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

[Wanpeng Li]

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,
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