On Fri, 23 Jul 2021 at 13:41, 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 1-4 add tertiary processor-based VM-execution support > framework. > > Patch 5 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 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 > 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 consuming in xAPIC mode and x2APIC mode > respectly. > > KMV unittest:vmexit/ipi, 2 vCPU, AP was modified to run in idle loop > instead of 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% > > -------------------------------------- > 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 Improve the performance for unicast ipi is as expected, however, I wonder whether the broadcast performance is worse than PV IPIs/Thomas's IPI shorthands(IPI shorthands are supported by upstream linux apic/x2apic driver). The hardware acceleration is not always outstanding on AMD(https://lore.kernel.org/kvm/CANRm+Cx597FNRUCyVz1D=B6Vs2GX3Sw57X7Muk+yMpi_hb+v1w@xxxxxxxxxxxxxx/), how about your Intel guys? Please try a big VM at least 96 vCPUs as below or more bigger. > > 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 > about 9.73% time saving on average out of 15 test runs when IPIv is > enabled. > > 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% Good to know this. Wanpeng
