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

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Date:   Mon, 10 Jun 2019 18:11:00 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v3 0/3] KVM: Yield to IPI target if necessary

On Mon, Jun 10, 2019 at 04:34:20PM +0200, Radim Krčmář 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.