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

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Date:	Mon, 07 Nov 2011 17:22:21 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Avi Kivity <avi@...hat.com>
Cc:	Gleb Natapov <gleb@...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, 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.

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