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Date: Fri, 23 Jul 2021 14:15:53 +0800
From: Zeng Guang <guang.zeng@...el.com>
To: Wanpeng Li <kernellwp@...il.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>,
"Luck, Tony" <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>,
"Huang, Kai" <kai.huang@...el.com>,
the arch/x86 maintainers <x86@...nel.org>,
LKML <linux-kernel@...r.kernel.org>,
"Hu, Robert" <robert.hu@...el.com>,
"Gao, Chao" <chao.gao@...el.com>
Subject: Re: [PATCH 0/5] IPI virtualization support for VM
On 7/19/2021 3:37 PM, Wanpeng Li wrote:
> 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.
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
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