linux-kernel - Re: [PATCH 1/2] KVM: X86: Single target IPI fastpath

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Date:   Mon, 11 Nov 2019 22:59:14 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH 1/2] KVM: X86: Single target IPI fastpath

On 09/11/19 08:05, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> This patch tries to optimize x2apic physical destination mode, fixed delivery
> mode single target IPI by delivering IPI to receiver immediately after sender
> writes ICR vmexit to avoid various checks when possible.
> 
> Testing on Xeon Skylake server:
> 
> The virtual IPI latency from sender send to receiver receive reduces more than
> 330+ cpu cycles.
> 
> Running hackbench(reschedule ipi) in the guest, the avg handle time of MSR_WRITE
> caused vmexit reduces more than 1000+ cpu cycles:
> 
> Before patch:
> 
>   VM-EXIT    Samples  Samples%     Time%    Min Time    Max Time   Avg time
> MSR_WRITE    5417390    90.01%    16.31%      0.69us    159.60us    1.08us
> 
> After patch:
> 
>   VM-EXIT    Samples  Samples%     Time%    Min Time    Max Time   Avg time
> MSR_WRITE    6726109    90.73%    62.18%      0.48us    191.27us    0.58us

Do you have retpolines enabled?  The bulk of the speedup might come just
from the indirect jump.

Paolo