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Message-ID: <CANRm+CyinezdL4udNv1fkCymCUdOjG7wjBPKsbcMTVw0pAbcjA@mail.gmail.com>
Date: Mon, 19 Jul 2021 15:37:35 +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 0/5] IPI virtualization support for VM
On Mon, 19 Jul 2021 at 15:26, Zeng Guang <guang.zeng@...el.com> wrote:
>
> On 7/16/2021 5:25 PM, Wanpeng Li wrote:
> > On Fri, 16 Jul 2021 at 15:14, 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 1-3 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.
Wanpeng
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