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SubjectRe: [RFC] KVM: x86: SVM: don't expose PV_SEND_IPI feature with AVIC
On Mon, 8 Nov 2021 at 20:48, Kele Huang <huangkele@bytedance.com> wrote:
>
> Currently, AVIC is disabled if x2apic feature is exposed to guest
> or in-kernel PIT is in re-injection mode.
>
> We can enable AVIC with options:
>
> Kmod args:
> modprobe kvm_amd avic=1 nested=0 npt=1
> QEMU args:
> ... -cpu host,-x2apic -global kvm-pit.lost_tick_policy=discard ...
>
> When LAPIC works in xapic mode, both AVIC and PV_SEND_IPI feature
> can accelerate IPI operations for guest. However, the relationship
> between AVIC and PV_SEND_IPI feature is not sorted out.
>
> In logical, AVIC accelerates most of frequently IPI operations
> without VMM intervention, while the re-hooking of apic->send_IPI_xxx
> from PV_SEND_IPI feature masks out it. People can get confused
> if AVIC is enabled while getting lots of hypercall kvm_exits
> from IPI.
>
> In performance, benchmark tool
> https://lore.kernel.org/kvm/20171219085010.4081-1-ynorov@caviumnetworks.com/
> shows below results:
>
> Test env:
> CPU: AMD EPYC 7742 64-Core Processor
> 2 vCPUs pinned 1:1
> idle=poll
>
> Test result (average ns per IPI of lots of running):
> PV_SEND_IPI : 1860
> AVIC : 1390
>
> Besides, disscussions in https://lkml.org/lkml/2021/10/20/423
> do have some solid performance test results to this.
>
> This patch fixes this by masking out PV_SEND_IPI feature when
> AVIC is enabled in setting up of guest vCPUs' CPUID.

This is the second time in community you bytedance guys post patches
w/o evaluating ipi broadcast performance.
https://lore.kernel.org/all/CANRm+Cx597FNRUCyVz1D=B6Vs2GX3Sw57X7Muk+yMpi_hb+v1w@mail.gmail.com/T/#u

Wanpeng

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