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A Comparison of Software and Hardware Techniques for x86 Virtualization

A Comparison of Software and Hardware Techniques for x86 Virtualization. By Keith Adams and Ole Ageson VMWare. Presented by Mike Marty. The Renaissance of Virtualization. 1970s: virtual machines first used 1990s: x86 becomes prominent server platform No vertical integration in x86

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A Comparison of Software and Hardware Techniques for x86 Virtualization

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  1. A Comparison of Software and Hardware Techniques for x86 Virtualization By Keith Adams and Ole Ageson VMWare Presented by Mike Marty

  2. The Renaissance of Virtualization • 1970s: virtual machines first used • 1990s: • x86 becomes prominent server platform • No vertical integration in x86 • Lack of enterprise features in commodity OSs • 1999: VMWare first product to virtualize x86 • 2006: AMD and Intel offer hardware support

  3. Outline • Classic Virtualization • Software Virtualization • Intel/AMD Hardware Virtualization • Comparison and Results • Discussion

  4. Classic Virtualization • Popek and Goldberg’s Criteria: • Fidelity – run any software • Performance – run it fairly fast • Safety – VMM manages all hardware • Trap-and-Emulate only real solution until recently

  5. Trap-and-Emulate Virtualization 1. De-Privilege OS user mode apps OS kernel mode

  6. Trap-and-Emulate Virtualization 1. De-Privilege OS apps apps user mode OS OS virtual machine monitor kernel mode

  7. Trap-and-Emulate Virtualization 1. De-Privilege OS 2. Shadow structures and memory tracing shadow page table shadow page table apps apps user mode OS OS primary page table virtual machine monitor kernel mode

  8. Trap-and-Emulate cont. • Traps are expensive (~3000 cycles) • Many traps unavoidable • E.g., page faults • Important enhancements • “Paravirtualization” to reduce traps (e.g., Xen) • Hardware VM modes (e.g., IBM s370)

  9. Can x86 Trap and Emulate? • No • Even with 4 execution modes! • Key problem: dual-purpose instructions don’t trap • Classic Example: popf instruction • Same instruction behaves differently depending on execution mode • User Mode: changes ALU flags • Kernel Mode: changes ALU and system flags • Does not generate a trap in user mode

  10. Outline • Classic Virtualization • Software Virtualization • Intel/AMD Hardware Virtualization • Comparison and Results • Discussion

  11. Software Virtualization with VMWare • Binary translation! (mostly safe, user-mode) X86 X86

  12. VMWare’s Binary Translation • On-the-fly • Only need to translate OS code • Makes SPEC run fast by default • Most instruction sequences don’t change • Instructions that do change: • Indirect control flow: call/ret, jmp • PC-relative addressing • Privileged instructions • Adaptive Translation • “Innocent until proven guilty”

  13. Performance Advantages of BT • Translation sequences can be faster than native: • cli vs. vpu.flags.IF := 0 • Avoid privilege instruction traps • Example: rdtsc • Trap-and-emulate: 2030 cycles • Callout-and-emulate: 1254 cycles • BT emulation: 216 cycles (but TSC value is stale)

  14. Outline • Classic Virtualization • Software Virtualization • Intel/AMD Hardware Virtualization • Comparison and Results • Discussion

  15. AMD SVM and Intel VT • Extensions to x86-32 and x86-64 • Allows classic trap-and-emulate! • Hardware VM modes to reduce traps • Details: • VMCB – virtual machine control block • VMX mode for running guest OSs • Vmrun instruction to enter VMX mode • Many instructions and events cause VMX exits • Control fields in VMCB can change VMX exit behavior

  16. Hardware VM Example: syscall • VMM fills in VMCB exception table for Guest OS • Sets bit in VMCB notexit on syscall exception • VMM executes vmrun • Application invokes syscall • CPU  CPL #0, does not trap, vectors to VMCB exception table

  17. Software BT vs. Hardware VM • Binary Translation VMM: • Converts traps to callouts • Callouts faster than trapping • Faster emulation routine • VMM does not need to reconstruct state • Avoids callouts entirely • Hardware VMM: • Preserves code density • No precise exception overhead • Faster system calls

  18. Compute-bound Benchmarks Bottomline: little difference for SPEC

  19. Mixed Benchmarks Cygwin Make is SLOW! Process-based Thread-based Who Cares? Would Hardware VM do better for multithreaded database?

  20. Costs of Operations

  21. Nanobenchmarks

  22. VMWare Nanobenchmarks • syscall • Native/Hardware VMM: same • Software VMM: +2000 cycles • in • Native: 3209 cycles • Hardware VMM: 15826 cycles • Software VMM: 15x faster? • call/ret • Native/Hardware VMM: 11 cycles • Software VMM: 51 cycles

  23. Opportunities • Faster Microarchitecture implementations • Intel Core Duo already much faster than P4 • Hardware VMM algorithms • Software/Hardware Hybrid VMM • Hardware MMU • Virtualize DMA

  24. Catalysts for Discussion • Is BT really faster for things that matter? • Process-based Apache on Linux? • Who configures a system to constantly page? • VMWare is done, why bother with Hardware VM support? • Simplicity of VMM w/ Hardware support • New applications • Will next-gen hardware make binary translation unnecessary?

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