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

[Zeng Guang]

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.

> please post the IPI microbenchmark score w/ and w/o the
> patchset.(https://lore.kernel.org/kvm/20171219085010.4081-1-ynorov@xxxxxxxxxxxxxxxxxx),
> I found that the hardware acceleration is not always outstanding.
> https://lore.kernel.org/kvm/CANRm+Cx597FNRUCyVz1D=B6Vs2GX3Sw57X7Muk+yMpi_hb+v1w@xxxxxxxxxxxxxx
>
>      Wanpeng