VMMs / Hypervisors

Intel Corporation21 July 2008

INTEL CONFIDENTIAL

Agenda

- Xen internals- High level architecture- Paravirtualization- HVM

- Others - KVM- VMware- OpenVZ

INTEL CONFIDENTIAL

Xen Overview

INTEL CONFIDENTIAL

Xen Project bio

• Xen project was created in 2003 at the University of Cambridge Computer Laboratory in what's known as the Xen Hypervisor project

– Led by Ian Pratt with team members Keir Fraser, Steven Hand, and Christian Limpach. – This team along with Silicon Valley technology entrepreneurs Nick Gault and Simon Crosby founded

XenSource which was acquired by Citrix Systems in October 2007

• The Xen® hypervisor is an open source technology, developed collaboratively by the Xen community and engineers (AMD, Cisco, Dell, HP, IBM, Intel, Mellanox, Network Appliance, Novell, Red Hat, SGI, Sun, Unisys, Veritas, Voltaire, and of course, Citrix)

• Xen is licensed under the GNU General Public License• Xen supports Linux 2.4, 2.6, Windows and NetBSD 2.0

INTEL CONFIDENTIAL

Xen Components

A Xen virtual environment consists of several modules that provide the virtualization environment:

• Xen Hypervisor - VMM• Domain 0• Domain Management and Control• Domain User, can be one of:

– Paravirtualized Guest: the kernel is aware of virtualization– Hardware Virtual Machine Guest: the kernel runs natively

Hypervisor - VMM

Domain 0

Domain Managemen

t and Control

Domain UParavirtual Guest

Domain UParavirtual Guest

Domain UParavirtual Guest Domain U

HVM Guest

Domain UHVM Guest

Domain UHVM Guest

INTEL CONFIDENTIAL

Xen Hypervisor - VMM

The hypervisor is Xen itself. It goes between the hardware and the operating systems of the various domains. The hypervisor is responsible for:• Checking page tables• Allocating resources for new domains• Scheduling domains. • Booting the machine enough that it can start dom0. It presents the domains with a VirtualMachine that looks similar but not identical

to the native architecture.Just as applications can interact with an OS by giving it syscalls, domains interact

with the hypervisor by giving it hypercalls. The hypervisor responds by sending the domain an event, which fulfills the same function as an IRQ on real hardware.

A hypercall is to a hypervisor what a syscall is to a kernel.

INTEL CONFIDENTIAL

Restricting operations with Privilege RingsThe hypervisor executes privileged instructions, so it must be in the right place:• x86 architecture provides 4 privilege levels / rings• Most OSs were created before this implementation, so only 2 levels are used• Xen provides 2 modes:

– In x86 the applications are run at ring 3, the kernel at ring 1 and Xen at ring 0– In x86 with VT-x, the applications run at ring 3, the guest at ring non-root-0 and Xen at ring root-0 (-

1)

3

0

3

1

0

3

0

The Guest is moved to ring 1

Native Paravirtual x86

HVM x86

Applications Guest kernel (dom0 and dom U)

Hypervisor

The Hypervisor is moved to ring -1

INTEL CONFIDENTIAL

Domain 0Domain 0 is a Xen required Virtual Machine running a modified Linux kernel with

special rights to:• Access physical I/O devices

– Two drivers are included in Domain 0 to attend requests from Domain U PV or HVM guests

• Interact with the other Virtual Machines (Domain U)• Provides the command line interface for Xen daemonsDue to its importance, the minimum functionality should be provided and properly

securedSome Domain 0 responsibilities can be delegated to Domain U (isolated driver

domain)Domain 0

Network backend driver

Block backend driver

Communicates directly with the local networking hardware to process all virtual machines

requestsCommunicates with the local storage disk to read and write

data from the drive based upon Domain U requests

PV

HVM

Qemu-DMSupports HVM Guests for

networking and disk access requests

INTEL CONFIDENTIAL

Domain Management and Control - DaemonsThe Domain Management and Control is composed of Linux daemons and tools:• Xm

– Command line tool and passes user input to Xend through XML RPC

• Xend– Python application that is considered the system manager for the Xen environment

• Libxenctrl– A C library that allows Xend to talk with the Xen hypervisor via Domain 0 (privcmd driver delivers

the request to the hypervisor)

• Xenstored– Maintains a registry of information including memory and event channel links between Domain 0

and all other Domains

• Qemu-dm– Supports HVM Guests for networking and disk access requests

INTEL CONFIDENTIAL

Domain U – Paravirtualized guests

The Domain U PV Guest is a modified Linux, Solaris, FreeBSD or other UNIX system that is aware of virtualization (no direct access to hardware)

No rights to directly access hardware resources, unless especially grantedAccess to hardware through front-end drivers using the split device driver modelUsually contains XenStore, console, network and block device driversThere can be multiple Domain U in a Xen configuration

Domain U - PV

Network front-end driver

Block front-end driver

Communicates with the Network backend driver in Domain 0

Communicates with the Block backend driver in Domain 0

Console driver

XenStore driver

Similar to a registry

INTEL CONFIDENTIAL

Domain U – HVM guests

The Domain U HVM Guest is a native OS with no notion of virtualization (sharing CPU time and other VMs running)

An unmodified OS doesn’t support the Xen split device driver, Xen emulates devices by borrowing code from QEMU

HVMs begin in real mode and gets configuration information from an emulated BIOS

For an HVM guest to use Xen features it must use CPUID and then access the hypercall page

Domain U - HVM

Xen virtual firmware

Simulates the BIOS for the unmodified operating system to

read it during startup

12Software and Solutions Group INTEL CONFIDENTIAL

• In an operating system with protected memory, each application has it own address space. A hypervisor has to do something similar for guest operating systems.

• The triple indirection model is not necessarily required but it is more convenient from the performance point of view and modifications needed in the guest kernel.

• If the guest kernel needs to know anything about the machine pages, it has to use the translation table provided by the shared info page (rare)

Pseudo-Physical to Memory Model

… …

… …

… …

Application

Kernel

Hypervisor

Virtual

Pseudo-physical

Machine

13Software and Solutions Group INTEL CONFIDENTIAL

Pseudo-Physical to Memory Model

There are variables at various places in the code identified as MFN, PFN, GMFN and GPFN

PFN (Page Frame Number) It means “some kind of page frame number”. The exact meaning depends on the context

MFN (Machine frame number) Number of a page in the (real) machine’s address space

GPFN (Guest page frame number)

These are page frames in the guest’s address space. These page addresses are relative to the local page tables

GMFN (Guest machine frame number)

This refers to either a MFN or a GPFN, depending on the architecture

INTEL CONFIDENTIAL

Virtual Ethernet interfaces

Xen creates, by default, seven pair of "connected virtual ethernet interfaces" for use by dom0

For each new domU, it creates a new pair of "connected virtual ethernet interfaces", with one end in domU and the other in dom0

Virtualized network interfaces in domains are given Ethernet MAC addresses (by default xend will select a random address)

The default Xen configuration uses bridging (xenbr0) within domain 0 to allow all domains to appear on the network as individual hosts

INTEL CONFIDENTIAL

The Virtual Machine lifecycle

OFF RUNNING

SUSPENDED

PAUSED

Turn on

Turn off

Resume

PauseStart (pause

d)

Stop

Turn off

WakeSleep

Migrate

Xen provides 3 mechanisms to boot a VM:- Booting from scratch (Turn on)- Restoring the VM from a previously saved state (Wake)- Clone a running VM (only in XenServer)

INTEL CONFIDENTIAL

A project: provide VMs for instantaneous/isolated executionGoal: determine a mechanism for instantaneous execution of applications in

sandboxed VMsApproach:• Analyze current capabilities in Xen• Implement a prototype that addresses the specified goal: VM-Pool

Technical specification of HW and SW used:• Intel® Core™ Duo T2400 @ 1.83GHz 1828 MHz• Motherboard Properties

– Motherboard ID: <DMI>– Motherboard Name: LENOVO 1952D89

• 2048 MB RAM• Software:

– Linux Fedora Core 8 Kernel 2.6.3.18– Xen 3.1– For the Windows images Windows XP SP2

INTEL CONFIDENTIAL

Analyzing Xen spawning mechanisms

# of CPU Time

1 93.5 sec

2 79 sec

# of CPU Time

1 19.5 sec

2 22 sec

VM RAMSize

Image in Hard Disk Image in RAM Disk

256 MB 16 sec 13 sec

512 MB 21 sec 15 sec

• Booting from scratch

HVM WinXP varying the #CPU

PV Fedora 8 varying the #CPU

• Restoring from a saved state

HVM WinXP 4GB disk / 1CPU

PV Fedora 8 varying the #CPU

VM RAMSize

HDD RAM disk

256 MB 15 sec 9 sec

512 MB 23 sec 16 sec

1024 MB 37 sec 29 sec

• Cloning a running VM

HVM WinXP 4GB disk / 1CPU

Image size Time

2 GB 145 sec

4 GB 220 sec

8 GB 300 sec

INTEL CONFIDENTIAL

Dynamic Spawning with a VM-Pool

• To have a pool of virtual machines already booted and ready for execution, but in a “paused” state

• These virtual machines keep their RAM but they don’t use processor time, interrupts and other resources

Simple interface defined:• get: retrieves and unpauses a virtual machine from the pool • release: gives back a virtual machine to the pool and restarts the VM

High level description:VM Pool

Set of free Virtual Machines

VMM

VM1 VM2 VM3 VM4

VM Pool Manager

External interfaceto listen forrequests

INTEL CONFIDENTIAL

Results with the VM-Pool

• The VM is ready to run in less than half a second (~350 milliseconds)

• Preferred spawning method is resuming, although it uses additional disk storage

VMPool Initialization Time

0

50

100

150

200

250

300

VM Booting Mode

Seconds

Fromscratch

Resume

Initialization time - from scratch 265±21 seconds

Initialization time - resume 52±1 seconds

Get operation 341 milliseconds

Release operation - from scratch 110±21 seconds

Release operation - resume 30±2 seconds

INTEL CONFIDENTIAL

Virtual Machines SchedulingThe hypervisor is responsible for ensuring that every running guest receives some

CPU time.Most used scheduling mechanisms in Xen:• Simple Earliest Deadline First – SEDF (being deprecated):

– Each domain runs for an n ms slice every m ms (n and m are configured per-domain)

• Credit Scheduler:– Each domain has a couple of properties: a cap and a weight– Weight: determines the share of the physical CPU time that the domain gets, weights are relative to

each other– Cap: represents the maximum, it’s an absolute value– Default work-conserving; if no other VMs needs to use CPU, then the running one will be given

more time to execute– Uses a fixed-size 30ms quantum, and ticks every 10 ms

Xen provides a simple abstract interface to schedulers:struct scheduler {

char *name; /* full name for this scheduler */

char *opt_name; /* option name for this scheduler */

unsigned int sched_id; /* ID for this scheduler */

void (*init) (void);

int (*init_domain) (struct domain *);

void (*destroy_domain) (struct domain *);

int (*init_vcpu) (struct vcpu *);

void (*destroy_vcpu) (struct vcpu *);

void (*sleep) (struct vcpu *);

void (*wake) (struct vcpu *);

struct task_slice (*do_schedule) (s_time_t);

int (*pick_cpu) (struct vcpu *);

int (*adjust) (struct domain *, struct xen_domctl_scheduler_op *);

void (*dump_settings) (void);

void (*dump_cpu_state) (int);

};

INTEL CONFIDENTIAL

Xen Para-Virtual functionality

INTEL CONFIDENTIAL

We’ll review the PV mechanisms that support this architecture:- Kernel Initialization- Hypercalls creation- Event channels- XenStore (some kind of registry) - Memory transfers between VMs- Split device drivers

Paravirtualized architecture

Hypervisor

Domain 0

Backend

device driver

Paravirtual Guest

Real device driver

Frontend

device driver

HardwareBlock devices

Shared Ring

Buffers

INTEL CONFIDENTIAL

Initial information for booting a PV OS

• First things the OS needs to know when boots:– Available RAM, connected peripherals, access to the machine clock.

• An OS running on a PV Xen environment does not have access to real firmware– The information required is provided by the SHARED INFO PAGES.

• The “domain builder” is in charge of mapping the shared info pages in the guest’s address space prior its boot.– Example: launching dom0 in a i386 architecture:

• Refer to function construct_dom0 in xen/arch/x86/domain_build.c

• The shared info pages does not completely replace a BIOS– The console device is available via the start info page for debugging purposes;

debugging output from the kernel should be available as early as possible.– Other devices must be found using the XenStore

INTEL CONFIDENTIAL

The start info page• The start info page is loaded in the guest’s address space at boot

time. The way this page is transferred is architecture-dependent; x86 uses the ESI register.

• The content of this page is defined by the C structure start_info which is declared in xen/include/public/xen.h

• A portion of the fields in the start info page are always available for the guest domain and are updated every time the virtual machine is resumed because some of them contain machine addresses (subject to change

INTEL CONFIDENTIAL

start_info structure overview

struct start_info { /* THE FOLLOWING ARE FILLED IN BOTH ON INITIAL BOOT AND ON RESUME. */ char magic[32]; /* "xen-<version>-<platform>". */ unsigned long nr_pages; /* Total pages allocated to this domain. */ unsigned long shared_info; /* MACHINE address of shared info struct. */ uint32_t flags; /* SIF_xxx flags. */ xen_pfn_t store_mfn; /* MACHINE page number of shared page. */ uint32_t store_evtchn; /* Event channel for store communication. */ union { struct { xen_pfn_t mfn; /* MACHINE page number of console page. */ uint32_t evtchn; /* Event channel for console page. */ } domU; struct { uint32_t info_off; /* Offset of console_info struct. */ uint32_t info_size; /* Size of console_info struct from start.*/ } dom0; } console; /* THE FOLLOWING ARE ONLY FILLED IN ON INITIAL BOOT (NOT RESUME). */ unsigned long pt_base; /* VIRTUAL address of page directory. */ unsigned long nr_pt_frames; /* Number of bootstrap p.t. frames. */ unsigned long mfn_list; /* VIRTUAL address of page-frame list. */ unsigned long mod_start; /* VIRTUAL address of pre-loaded module. */ unsigned long mod_len; /* Size (bytes) of pre-loaded module. */ int8_t cmd_line[MAX_GUEST_CMDLINE];}; typedef struct start_info start_info_t;

INTEL CONFIDENTIAL

start_info fields

char magic[32]; /*"xen-<version>-platform>"*/

• The magic number is the first thing the guest domain must check from its start info page. – If the magic string does not start with “xen-” something is seriously

wrong and the best thing to do is abort.– Also, minor and major versions must be checked in order to determine if

the guest kernel had been tested in this Xen version.

unsigned long nr_pages; /*Total pages allocated to this domain.*/

• The amount of available RAM is determined by this field. It contains the number of memory pages available to the domain.

INTEL CONFIDENTIAL

start_info fields (2)

unsigned long shared_info; /*MACHINE address of shared info struct.*/

• Contains the address of the machine page where the shared info structure is. The guest kernel should map it to retrieve useful information for its initialization process.

uint32_t flags; /* SIF_xxx flags.*/

• Contains any optional settings for this domain. (defined in xen.h)– SIF_PRIVILEGED, SIF_INITDOMAIN

xen_pfn_t store_mfn; /* MACHINE page number of shared page.*/

• Machine address of the shared memory page used for communication with the XenStore.

uint32_t store_evtchn; /* Event channel for store communication.*/

• Event channel used for notifications.

INTEL CONFIDENTIAL

start_info fields (3)

union {

struct {

xen_pfn_t mfn; /* MACHINE page number of console page.*/

uint32_t evtchn; /* Event channel for console page.*/

}domU;

struct {

uint32_t info_off; /*Offset of console_info struct. */

uint32_t info_size; /*Size of console_info struct from start.*/

}dom0;

}console; • Domain 0 guests uses the dom0 part, which contains the memory

offset and size of the structure used to define the Xen console.• For unprivileged domains the domU part of the union is used .The

fields in this represent a shared memory page and event channel used to identify the console device.

INTEL CONFIDENTIAL

The shared Info Page

• The shared info contains information that is dynamically updated as the system runs.

• It is explicitly mapped by the guest. • The content of this page is defined by the C structure shared_info which is

declared in xen/include/public/xen.h

shard_info_t

vcpu_info[]

evtchn_pending

evtchn_mask

wc_version

wc_sec

wc_nsec

arch

arch_shared_info_t

max_pfn

pfn_to_mfn_frame_list_list

arch_vcpu_t

cr2

pad

arch_time_info_t

version

pad0

tsc_timestamp

system_time

tsc_to_system_mul

tsc_shift

pad1

vcpu_info_t

evtchn_upcall_pending

evtchn_upcall_mask

evtchn_pending_sel

arch

time

INTEL CONFIDENTIAL

shared_info fields

struct vcpu_info_t vcpu_info[MAX_VIRT_CPUS]

• This array contains one entry per virtual CPU assigned to the domain. Each array element is a vcpu_info_t structure containing CPU specific information:

– Each virtual CPU has 3 flags relating to virtual interrupts (asynchronously delivered events).• uint8_t evtchn_upcall_pending: it is used by Xen to notify the running

system that there are upcalls currently waiting for delivery on this virtual CPU.• uint8_t evtchn_upcall_mask: This is the mask for the previous field. This mask

prevents any upcalls being delivered to the running virtual CPU.• unsigned long evtchn_pending_sel: Indicates which event is waiting. The

event bitmap is an array of machine words, and this value indicates which word in the evtchn_pending field of the parent structure indicates the raised event.

– arch• Architecture-specific information.

– On x86, this include the virtual CR2 register, that contains the linear address of the last page fault, but can only be read from ring 0. This is automatically copied by the hypervisor’s page fault handler before raising the event with the guest domain.

– time• This field, along with a number of fields sharing the wc_ (wall clock) prefix, is

used to implement time keeping in paravirtualized Xen guests.

INTEL CONFIDENTIAL

shared_info fields (2)

unsigned long evtchn_pending[sizeof(unsigned long) * 8];

• This is a bitmap that indicates which event channels have events waiting. (256 and 512 event channels on a 32 and 64-bit systems respectively)– Bits are set by the hypervisor and cleared by the guest.

unsigned long evtchn_mask[sizeof(unsigned long) * 8];

• This bitmap determines whether an event on a particular channel should be delivered asynchronously– Every time an event is generated, the corresponding bit in evtchn_pending

is set to 1. If the corresponding bit in evtchn_mask is set to 0, the hypervisor issues an upcall and delivers the event asynchronously. This allows the guest kernel to switch between interrupt-driven and polling mechanisms on a per-channel basis.

struct arch_shared_info arch;

• On x86 arch the arch_shared_info structure contains two fields; max_pfn and pfn_to_mfn_frame_list_list related to pseudo-physical to machine memory mapping.

INTEL CONFIDENTIAL

An exercise: The simplest Xen kernel

INTEL CONFIDENTIAL

• Bootstrap– Each Xen guest kernel must start with a section __xen_guest for the bootloader, with key-value

pairs• GUEST_OS: name of the running kernel• XEN_VER: specifies the Xen version for which the guest was implemented• VIRT_BASE: guest’s address space this allocation is mapped (0 for kernels)• ELF_PADDR_OFFSET: value subtracted from addresses in ELF headers (0 for kernels)• HYPERCALL_PAGE: specifies the page number where the hypercall trampolines will be set• LOADER: special boot loaders (currently only generic is available)

– After mapping everything into memory at the right places, Xen passes control to the guest kernel• A trampoline is defined _start

– Clears the direction flag, sets up the stack and calls the kernel start passing the start info page address in the ESI register as a parameter

– A guest kernel is expected to setup handlers to receive events at boot time, otherwise the kernel is not able to respond to the outside world (it is ignored in the book’s example)

• Kernel.c– The start_kernel routine takes the start info page as the parameter (passed through the ESI)– The stack is reserved in this file, although it was referenced in bootstrap as well for creating the

trampoline routine– If the hypervisor was compiled with debugging, then the HYPERVISOR_console_io will send the

string to the console (otherwise the hypercall fails)

• Debug.h– The hypercall takes three arguments: the command (write), the length of the string and the string

pointer– The hypercall # is 18 (xen/include/public/xen.h)

The simplest Xen kernel

INTEL CONFIDENTIAL

Hypercalls

INTEL CONFIDENTIAL

Executing Privileged instructions from appsBecause guest kernels don’t run at ring 0 they’re not allowed to execute

privileged instructions, a mechanism is needed to execute them in the right ring, supose exit(0):

push dword 0mov eax, 1push eacint 80h

Application Application

Kernel

Kernel

HypervisorRing 0

Ring 1

Ring 2

Ring 3

Native Paravirtualized

System CallHypercallDirect System Call (Xen specific)

The Hypervisor

has the interrupts

table

INTEL CONFIDENTIAL

Replacing Privileged instructions with HypercallsUnmodified guests use privileged instructions which require transition to ring 0, causing

performance penalty if resolved by the hypervisorParavirtual guests replace their privilege instructions by hypercallsXen uses 2 mechanisms for hypercalls:1. An int 82h is used as the channel similar to system calls (deprecated after Xen 3.0)2. Issued indirectly using the hypercall page provided when the guest is started

For the second mechanism, macros are provided to write hypercalls#define _hypercall2(type, name, a1, a2) \

({ \

long __res, __ign1, __ign2; \

asm volatile ( \

"call hypercall_page + ("STR(__HYPERVISOR_##name)" * 32)"\

: "=a" (__res), "=b" (__ign1), "=c" (__ign2) \

: "1" ((long)(a1)), "2" ((long)(a2)) \

: "memory" ); \

(type)__res; \

})

A PV Xen guest uses the HYPERVISOR_sched_op function with SCHEDOP_yield argument instead of using the privileged instruction HLT, in order to relinquish CPU time to guests with running tasks

static inline int HYPERVISOR_sched_op(int cmd, void *arg)

{

return _hypercall2(int, sched_op, cmd, arg);

}

extras/mini-os/include/x86/x86_32/hypercall-x86_32.h, implemented at xen/common/schedule.c

INTEL CONFIDENTIAL

Event Channels

INTEL CONFIDENTIAL

Event Channels

Event channels are the basic primitive provided by Xen for event notifications, equivalent of a hardware interrupt valid for paravirtualized OSs

Events are one bit of information signaled by transitioning from 0 to 1• Physical IRQs: mapped from real IRQs used to communicate with hardware

devices• Virtual IRQs: similar to PIRQs, but related to virtual devices such as the timer,

debug console• Interdomain events: bidirectional interrupts that notify domains about certain

event• Intradomain events: special case of interdomain events

Hypervisor - VMM

Domain 0Domain

Management and Control

Domain UParavirtual Guest

Hardware

Event Channel driver

INTEL CONFIDENTIAL

Event Channel Interface

Guests configure the Event Channel and send interrupts by issuing a specific hypercall: HYPERVISOR_event_channel_op (...)

Guests are notified about pending events through callbacks installed during initialization, these events can be masked dynamicallyHYPERVISOR_set_callbacks(…)

Hypervisor - VMM

Domain 0Domain

Management and Control

Domain UParavirtual Guest

Hardware

Event Channel driver

HYPERVISOR_event_channel_opCallback

INTEL CONFIDENTIAL

HYPERVISOR_event_channel_op – 1/2

HYPERVISOR_event_channel_op(int cmd, void *arg) // defined at xen-3.1.0-src\linux-2.6-xen-sparse\include\asm-i386\mach-xen\asm\hypercall.h

• EVTCHNOP_alloc_unbound: Allocate a new event channel port, ready to be connected to by a remote domain

– Specified domain must exist– A free port must exist in that domain

• EVTCHNOP_bind_interdomain: Bind an event channel for interdomain communications– Caller domain must have a free port to bind.– Remote domain must exist.– Remote port must be allocated and currently unbound.– Remote port must be expecting the caller domain as the remote.

• EVTCHNOP_bind_virq: Allocate a port and bind a VIRQ to it– Caller domain must have a free port to bind.– VIRQ must be valid.– VCPU must exist.– VIRQ must not currently be bound to an event channel

• EVTCHNOP_bind_ipi: Allocate and bind a port for notifying other virtual CPUs.– Caller domain must have a free port to bind.– VCPU must exist.

• EVTCHNOP_bind_pirq: Allocate and bind a port to a real IRQ.– Caller domain must have a free port to bind.– PIRQ must be within the valid range.– Another binding for this PIRQ must not exist for this domain.

INTEL CONFIDENTIAL

HYPERVISOR_event_channel_op – 2/2

HYPERVISOR_event_channel_op(int cmd, void *arg) /* defined at xen-3.1.0-src\linux-2.6-xen-sparse\include\asm-i386\mach-xen\asm\hypercall.h */

• EVTCHNOP_close: Close an event channel (no more events will be received).– Port must be valid (currently allocated).

• EVTCHNOP_send: Send a notification on an event channel attached to a port.– Port must be valid.

• EVTCHNOP_status: Query the status of a port; what kind of port, whether it is bound, what remote domain is expected, what PIRQ or VIRQ it is bound to, what VCPU will be notified, etc.

– Unprivileged domains may only query the state of their own ports. – Privileged domains may query any port.

INTEL CONFIDENTIAL

Issuing event channel hypercalls

Structures defined at xen-3.1.0-src\xen\include\public\event_channel.hHypervisor handlers defined at xen-3.1.0-src\xen\common\event_channel.c

• Allocating an unbound event channelevtchn_alloc_unbound_t op;

op.dom = DOMID_SELF;

op.remote_dom = remote_domain; /* an integer representing the domain */

if(HYPERVISOR_event_channel_op(EVTCHOP_alloc_unbound, &op) != 0)

{

/* Error */

}

• Binding an event channel for interdomain communicationevtchn_bind_interdomain_t op;

op.remote_dom = remote_domain;

op.remote_port = remote_port;

if(HYPERVISOR_event_channel_op(EVTCHOP_bind_interdomain, &op) != 0)

{

/* Error */

}

INTEL CONFIDENTIAL

HYPERVISOR_set_callbacks

Hypercall to configure the notification handlers

HYPERVISOR_set_callbacks(unsigned long event_selector, unsigned long event_address,unsigned long failsafe_selector, unsigned long failsafe_address)

/* defined at xen-3.1.0-src\linux-2.6-xen-sparse\include\asm-i386\mach-xen\asm\hypercall.h */

• event_selector + event_address: make the callback address for notifications• failsafe_selector + failsafe_address: make the callback if anything goes wrong

with the event

Notifications can be prevented at a VCPU level or at an event level because they’re contained in the shared info page:

struct shared_info {…

struct vcpu_info vcpu_info[MAX_VIRT_CPUS] {…

uint8_t evtchn_upcall_mask;…};

unsigned long evtchn_mask[sizeof(unsigned long) * 8];

…};

INTEL CONFIDENTIAL

Setting the notifications handler

Handler and masks configuration/* Locations in the bootstrapping code */

extern volatile shared_info_t shared_info;

void hypervisor_callback(void);

void failsafe_callback(void);

static evtchn_handler_t handlers[NUM_CHANNELS];

void EVT_IGN(evtchn_port_t port, struct pt_regs * regs) {};

/* Initialise the event handlers */

void init_events(void)

{

/* Set the event delivery callbacks */

HYPERVISOR_set_callbacks(

FLAT_KERNEL_CS, (unsigned long)hypervisor_callback,

FLAT_KERNEL_CS, (unsigned long)failsafe_callback);

/* Set all handlers to ignore, and mask them */

for(unsigned int i=0 ; i<NUM_CHANNELS ; i++)

{

handlers[i] = EVT_IGN;

SET_BIT(i,shared_info.evtchn_mask[0]);

}

/* Allow upcalls. */

shared_info.vcpu_info[0].evtchn_upcall_mask = 0;

}

INTEL CONFIDENTIAL

Implementing the callback function/* Dispatch events to the correct handlers */

void do_hypervisor_callback(struct pt_regs *regs)

{

unsigned int pending_selector, next_event_offset;

vcpu_info_t *vcpu = &shared_info.vcpu_info[0];

/* Make sure we don't lose the edge on new events... */

vcpu->evtchn_upcall_pending = 0;

/* Set the pending selector to 0 and get the old value atomically */

pending_selector = xchg(&vcpu->evtchn_pending_sel, 0);

while(pending_selector != 0)

{/* Get the first bit of the selector and clear it */

next_event_offset = first_bit(pending_selector);

pending_selector &= ~(1 << next_event_offset);

unsigned int event;

/* While there are events pending on unmasked channels */

while(( event = (shared_info.evtchn_pending[pending_selector] & ~shared_info.evtchn_mask[pending_selector])) != 0)

{

/* Find the first waiting event */

unsigned int event_offset = first_bit(event);

/* Combine the two offsets to get the port */

evtchn_port_t port = (pending_selector << 5) + event_offset; /* 5 -> 32 bits */

/* Handle the event */

handlers[port](port, regs);

/* Clear the pending flag */

CLEAR_BIT(shared_info.evtchn_pending[0], event_offset);

}

}

}

Maps a bit with an index in the

callback matrix

INTEL CONFIDENTIAL

XenStore

INTEL CONFIDENTIAL

XenStore is a hierarchical namespace (similar to sysfs or Open Firmware) which is shared between domains

The interdomain communication primitives exposed by Xen are very low-level (virtual IRQ and shared memory)

XenStore is implemented on top of these primitives and provides some higher level operations (read a key, write a key, enumerate a directory, notify when a key changes value)

General FormatThere are three main paths in XenStore: • /vm - stores configuration information about domain • /local/domain - stores information about the domain on the local node (domid,

etc.) • /tool - stores information for the various tools

Detailed information at http://wiki.xensource.com/xenwiki/XenStoreReference

Xen Store

INTEL CONFIDENTIAL

• The ring buffer is a fairly standard lockless data structure for producer-consumer communications

• Xen uses free-running counters• Each ring contains two kinds of data, a request and a response, updated by

the two halves of the driver• Xen only allows responses to be written in a way that overwrites requests

Ring buffers for split driver model

Domain U Front End

Write Request

Read Response

Domain 0 Back End

Read Request

Write Response

Response Notification Events

Request Notification Events

DomU writes Request 1 DomU writes Request 2 Dom0 writes Response 1

DomU reads Response 1 Dom0 writes Response 2 DomU reads Response 2

INTEL CONFIDENTIAL

Xen delegates hardware support typically to Domain 0, and device drivers typically consist of four main components:

• The real driver• The back end split driver• A shared ring buffer (shared memory pages and events notification)• The front end split driver

Xen Split Device Driver Model (for PV guests)

Hypervisor

Domain 0

Backend

device driver

Paravirtual Guest

Real device driver

Frontend

device driver

HardwareBlock devices

Shared Ring

Buffers

INTEL CONFIDENTIAL

Xen HVM functionality

INTEL CONFIDENTIAL

Hardware Virtual Machines allow unmodified Operating Systems to run on Virtual EnvironmentsThis approach brings 2 kinds of problems:- For the unmodified OS, the VM must appear as a real PC- Hardware access

- To keep isolation device emulation must be provided from Domain 0- Provide direct assignment from a VM to a specific HW

Xen HVM

Domain 0

Qemu-dm

Domain U - HVM

Xen virtual firmware

Every HVM has a qemu-dm counterpartHandles networking and disk access from HVMBased in QEMU project

Virtual BIOS to provide standard start-upComposed of 3 payloads- Vmxassist: real mode emulator for VMX- VGA BIOS- ROM BIOS

INTEL CONFIDENTIAL

Xen Virtual firmware works as the front end driver in the split driver model

1. Guest issues a BIOS interrupt requesting data to be loaded from disk2. The virtual BIOS translates the call into a request to the block device3. The vBIOS interrupt is caught by QEMU-dm4. QEMU-dm emulates the hardware and translates that to the real hardware in Domain 05. The inverse process happens for the response

Xen QEMU-dm / Virtual firmware interaction

Domain 0

Qemu-dm

Domain U - HVM

Xen virtual firmware

INTEL CONFIDENTIAL

Once the domain builder is specified as “hvm”:1. Allocates and verifies memory for domain2. Loads the hvmloader as a kernel (setup_guest at xc_hvm_build.c)3. Initializes hypercalls table and verifies that Xen is active4. Copies BIOS image to 0x000F0000 created from Bochs

(tools/firmware/rombios)5. Discovers and sets up PCI devices6. Loads a VGA BIOS7. For Intel platforms, loads real-mode emulator for VMX

(tools/firmware/vmxassist)

HVM domain creation

INTEL CONFIDENTIAL

Support for hardware virtualization is done through an abstract interface defined at xen/include/asm-x86/hvmstruct hvm_function_table {

char *name;

void (*disable)(void);

int (*vcpu_initialise)(struct vcpu *v);

void (*vcpu_destroy)(struct vcpu *v);

void (*store_cpu_guest_regs)(struct vcpu *v, struct cpu_user_regs *r, unsigned long *crs);

void (*load_cpu_guest_regs)(struct vcpu *v, struct cpu_user_regs *r);

void (*save_cpu_ctxt)(struct vcpu *v, struct hvm_hw_cpu *ctxt);

int (*load_cpu_ctxt)(struct vcpu *v, struct hvm_hw_cpu *ctxt);

int (*paging_enabled)(struct vcpu *v);

int (*long_mode_enabled)(struct vcpu *v);

int (*pae_enabled)(struct vcpu *v);

int (*interrupts_enabled)(struct vcpu *v);

int (*guest_x86_mode)(struct vcpu *v);

unsigned long (*get_guest_ctrl_reg)(struct vcpu *v, unsigned int num);

unsigned long (*get_segment_base)(struct vcpu *v, enum x86_segment seg);

void (*get_segment_register)(struct vcpu *v, enum x86_segment seg, struct segment_register *reg);

void (*update_host_cr3)(struct vcpu *v);

void (*update_guest_cr3)(struct vcpu *v);

void (*update_vtpr)(struct vcpu *v, unsigned long value);

void (*stts)(struct vcpu *v);

void (*set_tsc_offset)(struct vcpu *v, u64 offset);

void (*inject_exception)(unsigned int trapnr, int errcode, unsigned long cr2);

void (*init_ap_context)(struct vcpu_guest_context *ctxt, int vcpuid, int trampoline_vector);

void (*init_hypercall_page)(struct domain *d, void *hypercall_page);

int (*event_injection_faulted)(struct vcpu *v);

};

HVM support in Xen

INTEL CONFIDENTIAL

The hvm_function_table is initialized at xen/arch/x86/hvm/vmx/vmx.c

The following routines store and load completely save the state of a CPU through the VMCS .store_cpu_guest_regs = vmx_store_cpu_guest_regs .load_cpu_guest_regs = vmx_load_cpu_guest_regs

This status copy is performed in a single instructionstruct vmcs_struct {

u32 vmcs_revision_id;

unsigned char data [0]; /* vmcs size is read from MSR */

};

Intel VT support in Xen

INTEL CONFIDENTIAL

KVM overview

INTEL CONFIDENTIAL

What is KVM?

• It’s a VMM built within the Linux kernel– The name stands for Kernel Virtual Machines– It is included in mainline Linux, as of 2.6.20

• It offers full-virtualization– Para-virtualization support is in alpha state

• It works *only* in platforms with hardware-assisted virtualization– Currently only Intel-VT and AMD-V – Recently also s390, PowerPC and IA64

• Decision taken to achieve a simple design– No need to deal with ring aliasing problem,– Nor excessive faulting avoidance– Nor guest memory management complexity– Etc

INTEL CONFIDENTIAL

Why KVM?

• Today’s hardware is becoming increasingly complex– Multiple HW threads on a core– Multiple cores on a socket– Multiple sockets on a system– NUMA memory models (on-chip memory controllers)

• Scheduling and memory management is becoming harder accordingly

• Great effort is required to program all this complexity in hypervisors– But an operating system kernel already handles this complexity– So why no reuse it?

• KVM makes use of all the fine-tuning work that has gone (and is going) into the Linux kernel, applying it to a virtualized environment

• Minimal footprint– Less than 10K lines of kernel code– Implemented as a Linux’s module

INTEL CONFIDENTIAL

How it works?

• A normal Linux process has two modes of execution: kernel and user– KVM adds a third mode: guest mode

• A virtual machine in KVM will be “seen” as a normal Linux process– A portion of code will run in user mode: performs I/O on behalf of the guest– A portion of code will run in guest mode: performs non-I/O guest code

guest mode

With its own 4 rings

INTEL CONFIDENTIAL

Key features

• Simpler design: Kernel+Userspace (vs. Hypervisor+Kernel+Userspace)– Avoids many context switches– Code reuse (today and tomorrow)– Easy management of VMs (standard process tools)

• Supports Qcow2 and Vmdk disk image formats– “Growable” formats (copy-on-write)– Saved state of a VM with X Mb of RAM takes less than X Mb of file space

• KVM skips RAM sectors mapped by itself• KVM uses the on-the-fly compression capability of Qcow2 and VMDK formats• I.e. an save state of a 384Mb’s Windows VM occupies ~40Mb

– Discard-on-write capability (read’s made from base image A, write’s goes to new image B)

• B will contain the differences from A performed by the VM• Later, B diff’s can be merged into A

• Advanced guest memory management– Increased VM density with KSM (under development)[3]

• KSM is a kernel module to save memory by searching and merging identical pages inside one or more memory areas

– Balloon driver as in Xen– Guest’s page swapping allowed

INTEL CONFIDENTIAL

Future trends

• Para-virtualization support (Windows & Linux)– virtio devices already included in Linux’s mainline as of 2.6.25

• Storage[4]

– Many similar guests cause a lot of duplicate storage– Current solution: baseline + delta images

• Delta degrades overtime (needs planning)• Disk-in-file is overheady

– Future:• Block-level deduplication

– Filesystem or block device looks for identical blocks ... and consolidates them– Btrfs being analyzed right now (has snapshots & reverse mappings)

• Hostfs + file-based deduplication– No more virtual block device. Guest filesystem is a host directory– Host can carry out file dedup in the background– Requires changes in guest

• Para-virtualized file systems (9P from IBM Research)[2]

– Easy way to maintain consistency between two guests sharing a block device R/W

– Provide a direct file system proxy mechanism built on top of the native host<->guest I/O transport, avoiding unnecessary network stack overhead

INTEL CONFIDENTIAL

Future trends (2)

• Containers & Isolation (reduce the impact of one guest on others)– Memory containers

• Account each page to its container• Allows preferentially swapping some guests

– I/O accounting (since I/O affects other guests)• Each I/O in flight is correctly accounted to initiating task• Important for I/O scheduling

• Device passthrough methods– Several competing options

• 1:1 mapping with Intel VT-d• Virtualization-capable devices with PCI SIG Single Root IOV• PVDMA• Userspace IRQ delivery

– Still to see which will become mainline

• VMs-AS-FILES– Cross-hypervisor virtualization containers to allow for transportability of VMs– OVF: Open Virtual Appliance Format[5]

• Cross platform guest support (QuickTransit technology[6])– I.e. a Solaris for Sparc running in an Intel platform

INTEL CONFIDENTIAL

VMware overview

INTEL CONFIDENTIAL

VMwareIn 1998, VMware created a solution to virtualize the x86 platform, creating the market for x86 virtualizationThe solution was a combination of binary translation and direct execution on the processor

Nonvirtualizable instructions are replaced with new sequences of instructions

User level code is directly executed on the processorEach VMM provides each VM with all the services of

the physical system, including a virtual BIOS, virtual devices and virtualized memory management

INTEL CONFIDENTIAL

VMware ESX architectureDatacenter-class virtualization platform used by many enterprise customers for server consolidationRuns directly on a physical server having direct access to the physical hardware of the server

• Virtualization layer (VMM/VMKernel): implements the idealized hardware environment and virtualizes the physical hardware devices

• Resource Manager: partitions and controls the physical resources of the underlying machine

• Hardware interface components: enable hardware-specific service delivery• Service Console: boots the system, initiates execution of the virtualization layer and

resource manager, and relinquishes control to those layers• Add

– Virtual Center / Lab manager

INTEL CONFIDENTIAL

VMware default deploymentPrimary method of

interaction with virtual

infrastructure (console and GUI)

Virtualization layer that abstracts the

processor, memory, storage, and networking resources of the physical

host intomultiple virtual

machines

Centrallymanages the VMware ESX Server hosts

Organizes all theconfiguration data

for the virtual infrastructure environment

AuthorizesVirtualCenter Servers and

ESX Server hosts appropriately for the

licensingagreement

VI Client from theVirtualCenter Server or ESX Server hosts

INTEL CONFIDENTIAL

VMware for free

VMware provides freeware Server and Workstation virtualization solutions

• VMware Server:– Is a free desktop application that lets you run virtual machines on your Windows or

Linux PC– Lets you use host machine devices, such as CD and DVD drives, from the virtual

machine– Datasheet or FAQ page is available– Different Virtual Appliances are provided for free

• VMware Player:– Similar to VMware Server but limited to run pre-built virtual appliances

INTEL CONFIDENTIAL

OpenVZ overviewOperating System virtualization

INTEL CONFIDENTIAL

OpenVZ• OpenVZ is an open source server virtualization solution that creates multiple

isolated Virtual Private Servers (VPSs) or Virtual Environments (VEs) on a single physical server

• VPS perform and execute exactly like a stand-alone server for its users and applications as it can be rebooted independently

• All VPSs have their own set of processes and can run different Linux distributions, but all VPSs operate under the same kernel

• OpenVZ is the basis of Parallels/Virtuozzo Containers• Distinctive features:

– Operating System Virtualization– Network Virtualization– Resource Management– Templates

• Installation: http://wiki.openvz.org/Quick_installation• User documentation: http://download.openvz.org/doc/OpenVZ-Users-Guide.pdf

INTEL CONFIDENTIAL

OpenVZ Kernel

The OpenVZ kernel is a modified Linux kernel which adds the following functionality:

• Virtualization and isolation: enables many virtual environments within a single kernel

• Resource management: subsystem limits (and in some cases guarantees) resources such as CPU, RAM, and disk space on a per-VE basis

• Live Migration/Checkpointing: a process of “freezing” a VE, saving its complete state to a disk file, with the ability to “unfreeze” that state later

INTEL CONFIDENTIAL

OpenVZ Kernel Virtualization and IsolationEach Virtual Environment has its own set of virtualized/isolated resources, such

as:• Files

– System libraries, applications, virtualized /proc and /sys, virtualized locks, etc.

• Users and groups– Each VE has its own root user, as well as other users and groups.

• Process tree– A VE sees only its own set of processes, starting from init. PIDs are virtualized, so that the init PID is

1 as it should be.

• Network– Virtual network device, which allows the VE to have its own IP addresses, as well as a set of

netfilter (iptables) and routing rules.

• Devices– Devices are virtualized. In addition, any VE can be granted exclusive access to real devices like

network interfaces, serial ports, disk partitions, etc.

• IPC objects– Shared memory, semaphores, and messages.

INTEL CONFIDENTIAL

OVZ Resource Management

Resource management subsystem consists of three components:• Two-level disk quota:

– 1st level: Server administrator can set up per-VE disk quotas in terms of disk space and number of inodes

– 2nd level: VE administrator (VE root) uses standard UNIX quota tools to set up per-user and per-group disk quotas.

• “Fair” CPU 2 level scheduler:– 1st level: decides which VE to give the time slice to, taking into account the VE’s CPU priority and

limit settings– 2nd level: standard Linux scheduler decides which process in the VE to give the time slice to, using

standard process priorities.

• User Beancounters– This is a set of per-VE counters, limits, and guarantees– Set of about 20 parameters which are carefully chosen to cover all the aspects of VE operation, so

no single VE can abuse any resource which is limited for the whole computer and thus cause harm to other VEs

– The resources accounted and controlled are mainly memory and various in-kernel objects such as IPC shared memory segments, network buffers etc.

INTEL CONFIDENTIAL

OpenVZ Checkpointing and live migrationAllows the “live” migration of a VE to another physical serverA “frozen” VE and its complete state is saved to a disk file, then transferred to

another machineThis VE can then be “unfrozen” (restored) there (the whole process takes a few

seconds, and from the client’s point of view it looks not like a downtime, but rather a delay in processing, since the established network connections are also migrated)

Host Host

OpenVZ OpenVZ

Disk

VirtualEnv

VirtualEnv

Checkpoint

Live migration

INTEL CONFIDENTIAL

Backup

INTEL CONFIDENTIAL

Xen Terminology – 1/2Basicsguest operating system: An operating system that can run within the Xen environment. hypervisor: Code running at a higher privilege level than the supervisor code of its guest operating systems. virtual machine monitor ("vmm"): In this context, the hypervisor. domain: A running virtual machine within which a guest OS executes. domain0 ("dom0"): The first domain, automatically started at boot time. Dom0 has permission to control all hardware on the system, and is used to manage the

hypervisor and the other domains. unprivileged domain ("domU"): A domain with no special hardware access.

Approaches to Virtualizationfull virtualization: An approach to virtualization which requires no modifications to the hosted operating system, providing the illusion of a complete system of real

hardware devices. paravirtualization: An approach to virtualization which requires modifications to the operating system in order to run in a virtual machine. Xen uses

paravirtualization but preserves binary compatibility for user space applications.

Address SpacesMFN (machine frame number): Real host machine address; the addresses the processor understands. GPFN (guest pseudo-physical frame number): Guests run in an illusory contiguous physical address space, which is probably not contiguous in the machine address

space. GMFN (guest machine frame number): Equivalent to GPFN for an auto-translated guest, and equivalent to MFN for normal paravirtualised guests. It represents what

the guest thinks are MFNs. PFN (physical frame number): A catch-all for any kind of frame number. "Physical" here can mean guest-physical, machine-physical or guest-machine-physical.

Page TablesSPT (shadow page table): shadow version of a guest OSes page table. Useful for numerous things, for instance in tracking dirty pages during live migration. PAE: Intel's Physical Addressing Extensions, which enable x86/32 machines to address up to 64 GB of physical memory. PSE (page size extension): used as a flag to indicate that a given page is ahuge/super page (2/4 MB instead of 4KB).

x86 ArchitectureHVM: Hardware Virtual Machine, which is the full-virtualization mode supported by Xen. This mode requires hardware support, e.g. Intel's Virtualization Technology

(VT) and AMD's Pacifica technology. VT-x: full-virtualization support on Intel's x86 VT-enabled processors VT-i: full-virtualization support on Intel's IA-64 VT-enabled processors

Extracted from: http://wiki.xensource.com/xenwiki/XenTerminology

INTEL CONFIDENTIAL

Xen Terminology – 2/2Networking Infrastructurebackend: one half of a communication end point - interdomain communication is implemented using a frontend and backend device model interacting via event

channels. frontend: the device as presented to the guest; other half of the communication endpoint. vif: virtual interface; the name of the network backend device connected by an event channel to a network front end on the guest. vethN: local networking front end on dom0; renamed to ethN by xen network scripts in bridging mode (FIXME) pethN: real physical device (after renaming)

MigrationLive migration: A technique for moving a running virtual machine to another physical host, without stopping it or the services running on it.

SchedulingBVT: The Borrowed Virtual Time scheduler is used to give proportional fair shares of the CPU to domains. SEDF: The Simple Earliest Deadline First scheduler provides weighted CPU sharing in an intuitive way and uses realtime algorithms to ensure time guarantees.

Extracted from: http://wiki.xensource.com/xenwiki/XenTerminology

INTEL CONFIDENTIAL

Intel privileged instructionsSome of the system instructions (called “privileged instructions”) are protected from use by

application programs. The privileged instructions control system functions (such as the loading of system registers). They can be executed only when the CPL is 0 (most privileged). If one of these instructions is executed when the CPL is not 0, a general-protection exception (#GP) is generated. The following system instructions are privileged instructions (16):

• LGDT — Load GDT register.• LLDT — Load LDT register.• LTR — Load task register.• LIDT — Load IDT register.• MOV (control registers) — Load and store control registers.• LMSW — Load machine status word.• CLTS — Clear task-switched flag in register CR0.• MOV (debug registers) — Load and store debug registers.• INVD — Invalidate cache, without writeback.• WBINVD — Invalidate cache, with writeback.• INVLPG —Invalidate TLB entry.• HLT— Halt processor.• RDMSR — Read Model-Specific Registers.• WRMSR —Write Model-Specific Registers.• RDPMC — Read Performance-Monitoring Counter.• RDTSC — Read Time-Stamp Counter.

INTEL CONFIDENTIAL

http://bellard.org/qemu/qemu-tech.htmlA fast processor emulator using a portable dynamic emulator2 operating modes (add diagrams for each case):• Full system emulation• User mode emulationGeneric features:• User space only or full system emulation• Using dynamic translation to native code for reasonable speed• Working on x86 and PowerPC hosts. Being tested on ARM, Sparc32, Alpha and

S390• Self-modifying code support• Precise exceptions support• The virtual CPU is a library (libqemu) which can be used in other projectsQEMU full system emulation features:• QEMU can either use a full software MMU for maximum portability or use the

host system call mmap() to simulate the target MMU

QEMU Description - http://bellard.org/qemu/

INTEL CONFIDENTIAL

QEMU x86 target features: • Support for 16 bit and 32 bit addressing with segmentation. LDT/GDT and IDT

are emulated. VM86 mode is also supported to run DOSEMU• Support of host page sizes bigger than 4KB in user mode emulation• QEMU can emulate itself on x86

Current QEMU limitations: • No SSE/MMX support• No x86-64 support• IPC syscalls are missing• The x86 segment limits and access rights are not tested at every memory

access• On non x86 host CPUs, doubles are used instead of the non standard 10 byte

long doubles of x86 for floating point emulation to get maximum performances.

QEMU x86 emulation

INTEL CONFIDENTIAL

References• Intel® 64 and IA-32 Architectures - Software Developer’s Manual• http://wiki.xensource.com/xenwiki/XenArchitecture?action=AttachFile&do=get&target=X

en+Architecture_Q1+2008.pdf• http://wiki.xensource.com/xenwiki/XenArchitecture• http://www.xen.org/files/xensummit_4/Liguori_XenSummit_Spring_2007.pdf• http://wiki.xensource.com/xenwiki/XenTerminology• http://www.xen.org/xen/faqs.html• http://www.vmware.com/pdf/esx2_performance_implications.pdf• http://www.vmware.com/files/pdf/VMware_paravirtualization.pdf• http://download.openvz.org/doc/OpenVZ-Users-Guide.pdf• http://download.openvz.org/doc/openvz-intro.pdf• KVM project @ Sourceforge.net• Paravirtualized file systems, KVM Forum 2008.• Increasing Virtual Machine density with KSM, KVM Forum 2008.• Beyond kvm.ko, KVM Forum 2008.• Open-OVF: an OSS project around the Open Virtual Appliance format, KVM Forum 2008.• Cross platform guest support, KVM Forum 2008.