Re: [PATCH 0/5] IPI virtualization support for VM

[Wanpeng Li]

On Mon, 19 Jul 2021 at 15:26, Zeng Guang <guang.zeng@xxxxxxxxx> wrote:
>
> On 7/16/2021 5:25 PM, Wanpeng Li wrote:
> > On Fri, 16 Jul 2021 at 15:14, 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
> >> address" and "Last PID-pointer index" referenced by the VMCS. When "IPI
> >> virtualization" is enabled, processor emulateds following kind of writes
> >> 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.
> >>
> >> Patches 1-3 add tertiary processor-based VM-execution support
> >> framework.
> >>
> >> Patch 4 implement interrupt dispatch support in x2APIC mode with
> >> APIC-write VM exit. In previous platform, no CPU would produce
> >> APIC-write VM exit with exit qulification 300H when the "virtual x2APIC
> >> mode" VM-execution control was 1.
> >>
> >> Patch 5 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
> >> 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
> >> 22.21% and 15.98% cycles comsuming in xAPIC mode and x2APIC mode
> >> respectly.
> >>
> >> KMV unittest:vmexit/ipi, 2 vCPU, AP runs without halt to ensure no VM
> >> exit impact on target vCPU.
> >>
> >>                  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%
> > Commit a9ab13ff6e (KVM: X86: Improve latency for single target IPI
> > fastpath) mentioned that the whole ipi fastpath feature reduces the
> > latency from 4238 to 3293 around 22.3% on SKX server, why your IPIv
> > hardware acceleration is worse than software emulation? In addition,
>
> Actually this performance data was measured on the basis of fastpath
> optimization while cpu runs at base frequency.
>
> As a result, IPI virtualization could have extra 15.98% cost reduction
> over IPI fastpath process in x2apic mode.

I observed that adaptive advance lapic timer and adaptive halt-polling
will influence kvm-unit-tests/vmexit.flat IPI testing score, could you
post the score after disabling these features as commit a9ab13ff6e
(KVM: X86: Improve latency for single target IPI fastpath) mentioned？
In addition, please post the hackbench(./hackbench -l 1000000) and ipi
microbenchmark scores.

    Wanpeng