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

[Zeng Guang]

On 7/19/2021 3:37 PM, Wanpeng Li wrote:
> 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.

We modified unittest to make AP runing with idle loop instead of hlt . 
This eliminates the impact
from adaptive halt-polling. So far we don't observe the influence from 
adaptive advance lapic timer
by test either. vmexit/ipi test should not involve lapic timer.

We post the hackbench and ipi microbenchmark score in patch V2 for your 
reference.

Thanks.

>      Wanpeng