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

[Wanpeng Li <kernellwp@gmail.com>]

On Tue, 12 Nov 2019 at 09:33, Wanpeng Li <kernellwp@gmail.com> wrote:
>
> On Tue, 12 Nov 2019 at 05:59, Paolo Bonzini <pbonzini@redhat.com> wrote:
> >
> > 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.
>
> Adding 'mitigations=off' to the host grub parameter:
>
> Before patch:
>
>     VM-EXIT    Samples  Samples%     Time%    Min Time    Max Time   Avg time
> MSR_WRITE    2681713    92.98%    77.52%      0.38us     18.54us
> 0.73us ( +-   0.02% )
>
> After patch:
>
>     VM-EXIT    Samples  Samples%     Time%    Min Time    Max Time   Avg time
> MSR_WRITE    2953447    92.48%    62.47%      0.30us     59.09us
> 0.40us ( +-   0.02% )

Hmm, sender side less vmexit time is due to kvm_exit tracepoint is
still left in vmx_handle_exit, and ICR wrmsr is moved ahead, that is
why the time between kvm_exit tracepoint and kvm_entry tracepoint is
reduced.  But the virtual IPI latency still can reduce 330+ cycles.

    Wanpeng