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

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