Re: [PATCH v3 0/3] KVM: Yield to IPI target if necessary

From: Sean Christopherson
Date: Mon Jun 10 2019 - 21:15:27 EST

On Mon, Jun 10, 2019 at 04:34:20PM +0200, Radim KrÄmÃÅ wrote:
> 2019-05-30 09:05+0800, Wanpeng Li:
> > The idea is from Xen, when sending a call-function IPI-many to vCPUs,
> > yield if any of the IPI target vCPUs was preempted. 17% performance
> > increasement of ebizzy benchmark can be observed in an over-subscribe
> > environment. (w/ kvm-pv-tlb disabled, testing TLB flush call-function
> > IPI-many since call-function is not easy to be trigged by userspace
> > workload).
> Have you checked if we could gain performance by having the yield as an
> extension to our PV IPI call?
> It would allow us to skip the VM entry/exit overhead on the caller.
> (The benefit of that might be negligible and it also poses a
> complication when splitting the target mask into several PV IPI
> hypercalls.)

Tangetially related to splitting PV IPI hypercalls, are there any major
hurdles to supporting shorthand? Not having to generate the mask for
->send_IPI_allbutself and ->kvm_send_ipi_all seems like an easy to way
shave cycles for affected flows.