keith adams, ole agesen 1st october 2009 presented by chwa hoon sung, kang joon young a comparison...

Keith Adams, Ole Agesen

1st October 2009Presented by Chwa Hoon Sung, Kang Joon Young

A Comparison of Software and Hardware Techniques for x86 Vir-

tualization

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Virtualization

virtualization

3

Classic Virtualization

Software Virtualization

Hardware Virtualization

Comparison and Results

Discussion

Outline

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De-Privilege OS Executes guest operating systems directly but at lesser

privilege level, user-level

Classic Virtualization(Trap-and-Emulate)

OS

apps

kernel mode

user mode

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De-Privilege OS Executes guest operating systems directly but at lesser

privilege level, user-level

Classic Virtualization(Trap-and-Emulate)

OS

apps

kernel mode

user mode

virtual machine monitor

OS

apps

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Runs guest operating system deprivileged.

All privileged instructions trap into VMM.

VMM emulates instructions against virtual state.

Resumes direct execution from next guest instruction.

Trap-and-Emulate

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Architectural Obstacles Traps are expensive. (~3000 cycles) Many traps unavoidable. (e.g., page faults) Not all architectures support the trap-and-emulate.

(x86)

Classic Virtualization (Cont’d)

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Classic Virtualization(Popek & Goldberg)

System Virtualization

Trap-and-emulate

Software VMM Hardware VMM

Enhancement

Para-virtualization

(Xen)

Hardware Support for Virtualization

(Intel VT & AMD SVM)

Full-virtualization(VMware)

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Discussion

Outline

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Until recently, the x86 architecture has not permitted classical trap-and-emulate virtualization. Some privileged state is visible in user mode

Guest OS can observe that current privilege level (CPL) in code segment selection (%cs).

Not all privileged operations trap when run in user mode

Dual-purpose instructions don’t trap (popf).

Software VMMs for x86 have instead used binary translation of the guest code.


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Translates the kernel code to replace privileged instruc-tions with new sequences of instructions that have the intended effect on the virtual hardware.

The software VMM uses a translator with these proper-ties. Binary – input is machine-level code. Dynamic – occurs at runtime. On demand – code translated when needed for execution. System level – makes no assumption about guest code. Subsetting– translates from full instruction set to safe sub-

set. Adaptive – adjust code based on guest behavior to achieve

efficiency.

Binary Translation

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The translators input is full x86 instruction set, includ-ing all the privileged instructions; output is a safe subset of user-mode instructions

Binary Translation (Cont’d)

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Binary Translation

Translator

Guest Code

Translation

CacheCalloutsTC

Index

CPU EmulationRoutines

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vPC mov ebx, eax

cli

and ebx, ~0xfff

mov ebx, cr3

sti

ret

Guest Code

Straight-line code

Control flow

Basic Block

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vPC mov ebx, eax

cli

and ebx, ~0xfff

mov ebx, cr3

sti

ret

mov ebx, eax

call HANDLE_CLI

and ebx, ~0xfff

mov [CO_ARG], ebx

call HANDLE_CR3

call HANDLE_STI

jmp HANDLE_RET

start

Guest Code Translation Cache

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vPC mov ebx, eax

cli

and ebx, ~0xfff

mov ebx, cr3

sti

ret

mov ebx, eax

mov [CPU_IE], 0

and ebx, ~0xfff

mov [CO_ARG], ebx

call HANDLE_CR3

mov [CPU_IE], 1

test [CPU_IRQ], 1

jne

call HANDLE_INTS

jmp HANDLE_RET

start

Guest Code Translation Cache

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Avoid privilege instruction traps Example: rdtsc (read time-stamp counter) <- privileged

instruction Trap-and-emulate: 2030 cycles Callout-and-emulate: 1254 cycles (not TC) In TC emulation: 216 cycles

Performance Advantages of BT

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Discussion

Outline

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Recent x86 extension 1998 – 2005: Software-only VMMs using binary translation 2005: Intel and AMD start extending x86 to support virtu-

alization. First-generation hardware

Allows classical trap-and-emulate VMMs. Intel VT (Virtualization Technology) AMD SVM (Security Virtual Machine)

Performance VT/SVM help avoid BT, but not MMU ops. (actually

slower!) Main problem is efficient virtualization of MMU and I/O, Not

executing the virtual instruction stream.


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VMCB(Virtual Machine Control Block) in-memory data structure Contains the state of guest virtual

CPU. Modes

Non-root mode: guest OS runs at its intended privilege level(ring 0) (Not fully privileged)

Root mode: VMM is running at a new ring with an even higher privi-lege level(Fully privileged)

Instructions vmrun: transfers from root to non-

root mode. exit: transfers from non-root to

root mode.

New Hardware Features

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Intel VT-x Operations

Ring 0VMX RootMode

VMX Non-rootMode

. . .Ring 0

Ring 3

VM 1

Ring 0

Ring 3

VM 2

Ring 0

Ring 3

VM n

VMLAUNCHVM Run

VM Exit VMCB2

VMCBn

VMCB1

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Hardware VMM reduces guest OS dependency Eliminates need for binary translation Facilitates support for Legacy OS

Hardware VMM improves robustness Eliminates need for complex SW techniques Simpler and smaller VMMs

Hardware VMM improves performance Fewer unwanted (Guest VMM) transitions

Benefits of Hardware Virtu-alization

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Discussion

Outline

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BT tends to win in these areas: Trap elimination – BT can replace most traps with faster callouts.

Emulation Speed – callouts jump to predecoded emula-tion routine.

Callout avoidance – for frequent cases, BT may use in-TC emulation routines, avoiding even the callout cost.

The hardware VMM wins in these area: Code Density – since there is no translation. Precise exceptions – BT performs extra work to recover

guest state for faults. System calls – runs without VMM intervention.

Software VMM vs. Hard-ware VMM

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Software VMM – VMware Player 1.0.1

Hardware VMM – VMware implemented experimental hardware assisted VMM.

Host – HP workstation, VT-enabled 3.8 GHz Intel Pentium

All experiments are run natively, on software VMM and on Hardware-assisted VMM.

Experiments

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Test to stress process creation and destruction system calls, context switching, page table modifica-

tions, page faults, etc. Results – to create and destroy 40,000 processes

Host – 0.6 seconds Software VMM – 36.9 seconds Hardware VMM – 106.4 seconds

Forkwait Test

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Benchmark Custom guest OS – FrobOS Tests performance of single vir-

tualization sensitive operation Observations

Syscall (Native == HW << SW) Hardware – No VMM interven-

tion in so near native Software – traps

in (SW << Native << HW) Native – access a off-CPU regis-

ter Software VMM – translates “in”

into a short sequence of instruc-tions that access virtual model of the same.

Hardware – VMM intervention

Nanobenchmarks

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Observations (Cont’d) ptemod (Native << SW <<

HW) Both use shadowing tech-

nique to implement guest paging using traces for co-herency

PTE writes causes signifi-cant overhead compared to native

Nanobenchmarks (Cont’d)

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Discussion

Outline

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Microarchitecture Hardware overheads will shrink over time as implementations ma-

ture. Measurements on desktop system using a pre-production version

Intel’s Core microarchitecture. Hardware VMM algorithmic changes

Drop trace faults upon guest PTE modification, allowing temporary incoherency with shadow page tables to reduce costs.

Hybrid VMM Dynamically selects the execution technique

Hardware VMM’s superior system call performance Software VMM’s superior MMU performance

Hardware MMU support Trace faults, context switches and hidden page faults can be han-

dled effectively with hardware assistance in MMU virtualization.

Opportunities

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Hardware extensions allow classical virtualization on x86 architecture.

Extensions remove the need for Binary Translation and simplifies VMM design.

Software VMM fares better than Hardware VMM in many cases like context switches, page faults, trace faults, I/O.

New MMU algorithms might narrow the gap in per-formance.

Conclusion

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Benchmarks Apache ab benchmarking tool

– on Linux installation of Apache http server and on Windows installation

Tests I/O efficiency Observations

Both VMMs perform poorly Performance on Windows and

Linux differ widely Reason: Apache Configuration

Windows – single address space (less paging)

Hardware VMM is better Linux – multiple address spa-

ces (more paging) Software VMM is better

Server Workload

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Benchmark PassMark on Windows XP Pro-

fessional The suite of microbenchmarks

test various aspects of work-station performance.

Observations Large RAM test

Exhausts memory. (paging capabilities)

Intended to test paging capa-bility.

Software VMM is better. 2D Graphics test

Involves system calls. Hardware VMM is better.

Desktop-Oriented Workload

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Compilation times Linux kernel and Apache (on Cyg-win)

Observation Big compilation jobs – lots

of page faults. Software VMM is better in

handling page faults.

Less Synthetic Workload

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keith adams, ole agesen 1st october 2009 presented by chwa hoon sung, kang joon young a comparison...

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