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

[Paolo Bonzini]

On 09/11/19 08:05, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> 
> 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