linux-kernel - Re: Re: [PATCH] x86/kvm: Introduce boot parameter no-kvm-pvipi

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Date:   Thu, 21 Oct 2021 13:03:46 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     zhenwei pi <pizhenwei@...edance.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Wanpeng Li <wanpengli@...cent.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-doc@...r.kernel.org
Subject: Re: Re: [PATCH] x86/kvm: Introduce boot parameter no-kvm-pvipi

On Thu, 21 Oct 2021 at 11:05, zhenwei pi <pizhenwei@...edance.com> wrote:
>
>
> On 10/21/21 4:12 AM, Sean Christopherson wrote:
> > On Wed, Oct 20, 2021, Wanpeng Li wrote:
> >> On Wed, 20 Oct 2021 at 20:08, zhenwei pi <pizhenwei@...edance.com> wrote:
> >>>
> >>> Although host side exposes KVM PV SEND IPI feature to guest side,
> >>> guest should still have a chance to disable it.
> >>>
> >>> A typicall case of this parameter:
> >>> If the host AMD server enables AVIC feature, the flat mode of APIC
> >>> get better performance in the guest.
> >>
> >> Hmm, I didn't find enough valuable information in your posting. We
> >> observe AMD a lot before.
> >> https://lore.kernel.org/all/CANRm+Cx597FNRUCyVz1D=B6Vs2GX3Sw57X7Muk+yMpi_hb+v1w@mail.gmail.com/T/#u
> >
> > I too would like to see numbers.  I suspect the answer is going to be that
> > AVIC performs poorly in CPU overcommit scenarios because of the cost of managing
> > the tables and handling "failed delivery" exits, but that AVIC does quite well
> > when vCPUs are pinned 1:1 and IPIs rarely require an exit to the host.
> >
>
> Test env:
> CPU: AMD EPYC 7642 48-Core Processor
>
> Kmod args(enable avic and disable nested):
> modprobe kvm-amd nested=0 avic=1 npt=1
>
> QEMU args(disable x2apic):
> ... -cpu host,x2apic=off ...
>
> Benchmark tool:
> https://github.com/bytedance/kvm-utils/tree/master/microbenchmark/apic-ipi
>
> ~# insmod apic_ipi.ko options=5 && dmesg -c
>
>   apic_ipi: 1 NUMA node(s)
>   apic_ipi: apic [flat]
>   apic_ipi: apic->send_IPI[default_send_IPI_single+0x0/0x40]
>   apic_ipi: apic->send_IPI_mask[kvm_send_ipi_mask+0x0/0x10]
>   apic_ipi:     IPI[kvm_send_ipi_mask] from CPU[0] to CPU[1]
>   apic_ipi:             total cycles 375671259, avg 3756
>   apic_ipi:     IPI[flat_send_IPI_mask] from CPU[0] to CPU[1]
>   apic_ipi:             total cycles 221961822, avg 2219
>
>
> apic->send_IPI_mask[kvm_send_ipi_mask+0x0/0x10]
>    -> This line show current send_IPI_mask is kvm_send_ipi_mask(because
> of PV SEND IPI FEATURE)
>
> apic_ipi:       IPI[kvm_send_ipi_mask] from CPU[0] to CPU[1]
> apic_ipi:               total cycles 375671259, avg 3756
>    -->These lines show the average cycles of each kvm_send_ipi_mask: 3756
>
> apic_ipi:       IPI[flat_send_IPI_mask] from CPU[0] to CPU[1]
> apic_ipi:               total cycles 221961822, avg 2219
>    -->These lines show the average cycles of each flat_send_IPI_mask: 2219

Just single target IPI is not eough.

    Wanpeng