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SubjectRe: [PATCH 1/2] KVM: X86: Single target IPI fastpath
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On 09/11/19 08:05, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@tencent.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

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