linux-kernel - Re: [PATCHv2 2/9] KVM: Expose a version 2 architectural PMU to a guests

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Message-ID: <20111107162654.GL8670@redhat.com>
Date:	Mon, 7 Nov 2011 18:26:54 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Avi Kivity <avi@...hat.com>, kvm@...r.kernel.org,
	mtosatti@...hat.com, linux-kernel@...r.kernel.org, mingo@...e.hu,
	acme@...stprotocols.net, hpa <hpa@...or.com>
Subject: Re: [PATCHv2 2/9] KVM: Expose a version 2 architectural PMU to a
 guests

On Mon, Nov 07, 2011 at 05:22:21PM +0100, Peter Zijlstra wrote:
> On Mon, 2011-11-07 at 17:25 +0200, Avi Kivity wrote:
> > On 11/07/2011 05:19 PM, Gleb Natapov wrote:
> > > > 
> > > > note, this needs a fairly huge PMI skew to happen.
> > > > 
> > > No, it need not. It is enough to get exit reason as hlt instead of nmi
> > > for a vcpu to go to blocking state instead of reentering guest mode.
> > > Note that we do not check request flags in kvm_arch_vcpu_runnable().
> > 
> > Right.
> > 
> > If we had a guarantee about the maximum skew, we could add a check for
> > KVM_REQ_PMI in kvm_vcpu_block().
> 
> Right, it shouldn't be more than a few instructions since its NMIs we're
> talking about, but I'm not sure there's any really hard guarantees on,
> hardware folks would be able to say more.
> 
> Typically you can assert NMIs at instruction boundaries, but things like
> instruction fusing and a few 'special' insn can delay NMI delivery. then
> again, I'm clueless as to the actual implementation details of any of
> this stuff.
> 
> Also I'm not sure if there's any non-deterministic delays in the PMU
> event -> PMU overflow -> PMI raise path that could push you out into
> silly land.
> 
> So yeah, I think the proposed code is fine, although I think the comment
> can be improved by mentioning the vcpu hlt case.
> 
Will do.

--
			Gleb.
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