linux-kernel - Re: [RFC] KVM: x86: SVM: don't expose PV_SEND

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Message-ID: <CANRm+CxoxaiTcQvm98xs1wmxAhq_u5s-PrkF8Lm2-ovVJeFhuw@mail.gmail.com>
Date:   Tue, 16 Nov 2021 10:04:33 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Kele Huang <huangkele@...edance.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, chaiwen.cc@...edance.com,
        xieyongji@...edance.com, dengliang.1214@...edance.com,
        zhenwei pi <pizhenwei@...edance.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, kvm <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] KVM: x86: SVM: don't expose PV_SEND_IPI feature with AVIC

On Mon, 8 Nov 2021 at 20:48, Kele Huang <huangkele@...edance.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