linux-kernel - Re: [PATCH 0/5] KVM: x86: cleanup and minimal speedup for APICv

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Message-ID: <20161026215255.nscswmd7ynh2hyn4@redhat.com>
Date:   Thu, 27 Oct 2016 00:52:55 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        rkrcmar@...hat.com, yang.zhang.wz@...il.com, feng.wu@...el.com
Subject: Re: [PATCH 0/5] KVM: x86: cleanup and minimal speedup for APICv

On Fri, Oct 14, 2016 at 08:21:26PM +0200, Paolo Bonzini wrote:
> On some benchmarks (e.g. netperf with ioeventfd disabled), APICv
> posted interrupts turn out to be slower than interrupt injection via
> KVM_REQ_EVENT.
> 
> These patches save 40 cycles on kvm-unit-tests "inl" tests (1-2%), but I
> suspect the effect is bigger on workloads that inject interrupts heavily.
> 
> Patches 1-2 are micro-optimization of the APICv vmentry code.
> 
> The interesting one is patch 3.  The aim is for APICv to not use
> KVM_REQ_EVENT at all for interrupts, therefore turning APICv's weakness
> (having to look at PIR on every vmentry) into a strength (because
> checking PIR.ON is cheaper than processing KVM_REQ_EVENT).  I find that
> the new code overall makes more sense than the old one, and I like that
> more of the logic is contained within vmx_deliver_posted_interrupt and
> vmx_sync_pir_to_irr.  KVM_REQ_EVENT did protect from mistakes, so the new
> code is more delicate, but I still prefer that to duct tape. :)
> 
> Patches 4 and 5 are other cleanups.

FWIW, series

Acked-by: Michael S. Tsirkin <mst@...hat.com>

I still think we should make pir volatile but we
can argue about that later.

> Paolo
> 
> Paolo Bonzini (5):
>   KVM: x86: avoid atomic operations on APICv vmentry
>   KVM: x86: do not scan IRR twice on APICv vmentry
>   KVM: x86: do not use KVM_REQ_EVENT for APICv interrupt injection
>   KVM: x86: remove unnecessary sync_pir_to_irr
>   KVM: vmx: clear pending interrupts on KVM_SET_LAPIC
> 
>  arch/x86/kvm/lapic.c | 45 +++++++++++++++++++++++++++++----------------
>  arch/x86/kvm/lapic.h |  4 ++--
>  arch/x86/kvm/vmx.c   | 44 ++++++++++++++++++++++++++++++++------------
>  arch/x86/kvm/x86.c   | 45 +++++++++++++++++++++++++--------------------
>  4 files changed, 88 insertions(+), 50 deletions(-)
> 
> -- 
> 1.8.3.1