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

Actually, this is not the first attempt to add shortcut for MSR writes
which performance sensitive, the other effort is tscdeadline timer
from Isaku Yamahata, https://patchwork.kernel.org/cover/10541035/ ,
ICR and TSCDEADLINE MSR writes cause the main MSR write vmexits in our
product observation, multicast IPIs are not as common as unicast IPI
like RESCHEDULE_VECTOR and CALL_FUNCTION_SINGLE_VECTOR etc. As far as
I know, something similar to this patch has already been deployed in
some cloud companies private kvm fork.

Wanpeng

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