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Message-ID: <CANRm+CywPSiW=dniYEnUhYnK0NGGnnxV53AdC0goivndn6KR5g@mail.gmail.com>
Date: Fri, 23 Jul 2021 14:11:52 +0800
From: Wanpeng Li <kernellwp@...il.com>
To: Zeng Guang <guang.zeng@...el.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>,
Sean Christopherson <seanjc@...gle.com>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Wanpeng Li <wanpengli@...cent.com>,
Jim Mattson <jmattson@...gle.com>,
Joerg Roedel <joro@...tes.org>, kvm <kvm@...r.kernel.org>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Tony Luck <tony.luck@...el.com>,
Kan Liang <kan.liang@...ux.intel.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
"H. Peter Anvin" <hpa@...or.com>,
Kim Phillips <kim.phillips@....com>,
Jarkko Sakkinen <jarkko@...nel.org>,
Jethro Beekman <jethro@...tanix.com>,
Kai Huang <kai.huang@...el.com>,
"the arch/x86 maintainers" <x86@...nel.org>,
LKML <linux-kernel@...r.kernel.org>,
Robert Hu <robert.hu@...el.com>, Gao Chao <chao.gao@...el.com>
Subject: Re: [PATCH v2 0/6] IPI virtualization support for VM
On Fri, 23 Jul 2021 at 13:41, Zeng Guang <guang.zeng@...el.com> 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@caviumnetworks.com)
>
> 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@mail.gmail.com/),
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
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