linux-kernel - Re: [PATCH v2] KVM: x86: avoid redundant REQ

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Date:   Thu, 15 Dec 2016 17:56:58 +0300
From:   Roman Kagan <rkagan@...tuozzo.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
CC:     Radim Krčmář <rkrcmar@...hat.com>,
        Denis Plotnikov <dplotnikov@...tuozzo.com>,
        <den@...tuozzo.com>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] KVM: x86: avoid redundant REQ_EVENT

On Thu, Dec 15, 2016 at 03:32:45PM +0100, Paolo Bonzini wrote:
> 
> 
> On 15/12/2016 15:30, Radim Krčmář wrote:
> > 
> > One useless round of KVM_REQ_EVENT is not going change nested
> > performance by much and it is not the only thing we could improve wrt.
> > TPR ... I would just leave it for now and take care of it when we
> >  * don't to update PPR at all with APICv -- it is already correct
> >  * drop the KVM_REQ_EVENT with flex priority, because lower TPR cannot
> >    unmask an interrupt
> 
> I agree.  I still don't like the patch very much, because I feel like an
> explicit state machine ("can KVM_REQ_EVENT do anything?") would be more
> maintainable.

We all seem to share that feeling towards this patch :)  That's the
reason why it was baking here internally for a long time: Denis
discovered this scenario over a month ago while analyzing the
performance regressions in KVM against our proprietary hypervisor, but
pinning down a palatable and safe fix turned out to be a challenge.  

I think we did our best to stay safe; I agree that it ended up no so
beautiful indeed.

Roman.