Re: [PATCH v4 1/2] KVM: VMX: FIXED+PHYSICAL mode single target IPI fastpath

[Wanpeng Li]

On Thu, 21 Nov 2019 at 11:17, Wanpeng Li <kernellwp@xxxxxxxxx> wrote:
>
> From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
>
> ICR and TSCDEADLINE MSRs write cause the main MSRs write vmexits in our
> product observation, multicast IPIs are not as common as unicast IPI like
> RESCHEDULE_VECTOR and CALL_FUNCTION_SINGLE_VECTOR etc.
>
> This patch introduce a mechanism to handle certain performance-critical
> WRMSRs in a very early stage of KVM VMExit handler.
>
> This mechanism is specifically used for accelerating writes to x2APIC ICR
> that attempt to send a virtual IPI with physical destination-mode, fixed
> delivery-mode and single target. Which was found as one of the main causes
> of VMExits for Linux workloads.
>
> The reason this mechanism significantly reduce the latency of such virtual
> IPIs is by sending the physical IPI to the target vCPU in a very early stage
> of KVM VMExit handler, before host interrupts are enabled and before expensive
> operations such as reacquiring KVMâs SRCU lock.
> Latency is reduced even more when KVM is able to use APICv posted-interrupt
> mechanism (which allows to deliver the virtual IPI directly to target vCPU
> without the need to kick it to host).
>
> Testing on Xeon Skylake server:
>
> The virtual IPI latency from sender send to receiver receive reduces
> more than 200+ cpu cycles.

Testing by IPI microbenchmark(https://lkml.org/lkml/2017/12/19/141):

Normal IPI:           Improved 3%
Broadcast IPI:      Improved 5%

w/ --overcommit cpu-pm=on

Normal IPI:           Improved 14%
Broadcast IPI:      Improved 3.6%

    Wanpeng