Re: [PATCH v3 0/6] IPI virtualization support for VM

From: Jim Mattson
Date: Thu Aug 05 2021 - 13:03:20 EST

On Thu, Aug 5, 2021 at 8:38 AM Zeng Guang <guang.zeng@xxxxxxxxx> wrote:
>
> Current IPI process in guest VM will virtualize the writing to interrupt
> command register(ICR) of the local APIC which will cause VM-exit anyway
> on source vCPU. Frequent VM-exit could induce much overhead accumulated
> if running IPI intensive task.
>
> IPI virtualization as a new VT-x feature targets to eliminate VM-exits
> when issuing IPI on source vCPU. It introduces a new VM-execution
> control - "IPI virtualization"(bit4) in the tertiary processor-based
> VM-exection controls and a new data structure - "PID-pointer table
VM-execution
> address" and "Last PID-pointer index" referenced by the VMCS. When "IPI
> virtualization" is enabled, processor emulateds following kind of writes
emulates
> to APIC registers that would send IPIs, moreover without causing VM-exits.
> - Memory-mapped ICR writes
> - MSR-mapped ICR writes
> - SENDUIPI execution
>
> This patch series implement IPI virtualization support in KVM.
implements
>
> Patches 1-4 add tertiary processor-based VM-execution support
> framework.
>
> Patch 5 implement interrupt dispatch support in x2APIC mode with
implements
> APIC-write VM exit. In previous platform, no CPU would produce
> APIC-write VM exit with exit qulification 300H when the "virtual x2APIC
qualification
> mode" VM-execution control was 1.
>
> Patch 6 implement IPI virtualization related function including
> feature enabling through tertiary processor-based VM-execution in
> various scenario of VMCS configuration, PID table setup in vCPU creation
scenarios
> and vCPU block consideration.
>
> Document for IPI virtualization is now available at the latest "Intel
> Architecture Instruction Set Extensions Programming Reference".
>
> Document Link:
> https://software.intel.com/content/www/us/en/develop/download/intel-architecture-instruction-set-extensions-programming-reference.html
>
> We did experiment to measure average time sending IPI from source vCPU
> to the target vCPU completing the IPI handling by kvm unittest w/ and
> w/o IPI virtualization. When IPI virtualizatin enabled, it will reduce
virtualization
> 22.21% and 15.98% cycles consuming in xAPIC mode and x2APIC mode
> respectly.
respectively
>
> KMV unittest:vmexit/ipi, 2 vCPU, AP was modified to run in idle loop
KVM
> instead of halt to ensure no VM exit impact on target vCPU.
Are you going to post the kvm-init-test changes?
>
> Cycles of IPI
> xAPIC mode x2APIC mode
> test w/o IPIv w/ IPIv w/o IPIv w/ IPIv
> 1 6106 4816 4265 3768
> 2 6244 4656 4404 3546
> 3 6165 4658 4233 3474
> 4 5992 4710 4363 3430
> 5 6083 4741 4215 3551
> 6 6238 4904 4304 3547
> 7 6164 4617 4263 3709
> 8 5984 4763 4518 3779
> 9 5931 4712 4645 3667
> 10 5955 4530 4332 3724
> 11 5897 4673 4283 3569
> 12 6140 4794 4178 3598
> 13 6183 4728 4363 3628
> 14 5991 4994 4509 3842
> 15 5866 4665 4520 3739
> 16 6032 4654 4229 3701
> 17 6050 4653 4185 3726
> 18 6004 4792 4319 3746
> 19 5961 4626 4196 3392
> 20 6194 4576 4433 3760
>
> Average cycles 6059 4713.1 4337.85 3644.8
> %Reduction -22.21% -15.98%
>
> --------------------------------------
> IPI microbenchmark:
> (https://lore.kernel.org/kvm/20171219085010.4081-1-ynorov@xxxxxxxxxxxxxxxxxx)
>
> 2 vCPUs, 1:1 pin vCPU to pCPU, guest VM runs with idle=poll, x2APIC mode
For dedicated CPUs, we would just disable the MONITOR/MWAIT intercepts
and expose MONITOR/MWAIT to the guest. We would not recommend that
anyone use idle=poll.
> Result with IPIv enabled:
>
> Dry-run: 0, 272798 ns
> Self-IPI: 5094123, 11114037 ns
> Normal IPI: 131697087, 173321200 ns
> Broadcast IPI: 0, 155649075 ns
> Broadcast lock: 0, 161518031 ns
>
> Result with IPIv disabled:
>
> Dry-run: 0, 272766 ns
> Self-IPI: 5091788, 11123699 ns
> Normal IPI: 145215772, 174558920 ns
> Broadcast IPI: 0, 175785384 ns
> Broadcast lock: 0, 149076195 ns
>
>
> As IPIv can benefit unicast IPI to other CPU, Noraml IPI test case gain
Normal
> about 9.73% time saving on average out of 15 test runs when IPIv is
> enabled.
Can you share the CDFs?
>
> w/o IPIv w/ IPIv
> Normal IPI: 145944306.6 ns 131742993.1 ns
> %Reduction -9.73%
>
> --------------------------------------
> hackbench:
>
> 8 vCPUs, guest VM free run, x2APIC mode
> ./hackbench -p -l 100000
>
> w/o IPIv w/ IPIv
> Time: 91.887 74.605
> %Reduction: -18.808%
>
> 96 vCPUs, guest VM free run, x2APIC mode
> ./hackbench -p -l 1000000
>
> w/o IPIv w/ IPIv
> Time: 287.504 235.185
> %Reduction: -18.198%
>
> --------------------------------------
>
> v2 -> v3:
> 1. Misc change on tertiary execution control
> definition and capability setup
> 2. Alternative to get tertiary execution
> control configuration
>
> v1 -> v2:
> 1. Refine the IPIv enabling logic for VM.
> Remove ipiv_active definition per vCPU.
>
> Gao Chao (1):
> KVM: VMX: enable IPI virtualization
>
> Robert Hoo (4):
> x86/feat_ctl: Add new VMX feature, Tertiary VM-Execution control
> KVM: VMX: Extend BUILD_CONTROLS_SHADOW macro to support 64-bit
> variation
> KVM: VMX: Detect Tertiary VM-Execution control when setup VMCS config
> KVM: VMX: dump_vmcs() reports tertiary_exec_control field as well
>
> Zeng Guang (1):
> KVM: x86: Support interrupt dispatch in x2APIC mode with APIC-write VM
> exit
>
> arch/x86/include/asm/msr-index.h | 1 +
> arch/x86/include/asm/vmx.h | 11 +++
> arch/x86/include/asm/vmxfeatures.h | 5 +-
> arch/x86/kernel/cpu/feat_ctl.c | 11 ++-
> arch/x86/kvm/lapic.c | 9 ++-
> arch/x86/kvm/vmx/capabilities.h | 14 ++++
> arch/x86/kvm/vmx/evmcs.c | 2 +
> arch/x86/kvm/vmx/evmcs.h | 1 +
> arch/x86/kvm/vmx/posted_intr.c | 22 ++++--
> arch/x86/kvm/vmx/vmcs.h | 1 +
> arch/x86/kvm/vmx/vmx.c | 114 +++++++++++++++++++++++++++--
> arch/x86/kvm/vmx/vmx.h | 27 ++++---
> 12 files changed, 193 insertions(+), 25 deletions(-)
>
> --
> 2.25.1
>