Re: [PATCH] Enhance perf to collect KVM guest os statistics from host side

Sheng Yang
2010-03-17T09:28:44Z

On Wednesday 17 March 2010 10:34:33 Zhang, Yanmin wrote:
> On Tue, 2010-03-16 at 11:32 +0200, Avi Kivity wrote:
> > On 03/16/2010 09:48 AM, Zhang, Yanmin wrote:
> > > Right, but there is a scope between kvm_guest_enter and really running
> > > in guest os, where a perf event might overflow. Anyway, the scope is
> > > very narrow, I will change it to use flag PF_VCPU.
> >
> > There is also a window between setting the flag and calling 'int $2'
> > where an NMI might happen and be accounted incorrectly.
> >
> > Perhaps separate the 'int $2' into a direct call into perf and another
> > call for the rest of NMI handling. I don't see how it would work on svm
> > though - AFAICT the NMI is held whereas vmx swallows it.
> >
> > I guess NMIs
> > will be disabled until the next IRET so it isn't racy, just tricky.
>
> I'm not sure if vmexit does break NMI context or not. Hardware NMI context
> isn't reentrant till a IRET. YangSheng would like to double check it.

After more check, I think VMX won't remained NMI block state for host. That's
means, if NMI happened and processor is in VMX non-root mode, it would only
result in VMExit, with a reason indicate that it's due to NMI happened, but no
more state change in the host.

So in that meaning, there _is_ a window between VMExit and KVM handle the NMI.
Moreover, I think we _can't_ stop the re-entrance of NMI handling code because
"int $2" don't have effect to block following NMI.

And if the NMI sequence is not important(I think so), then we need to generate
a real NMI in current vmexit-after code. Seems let APIC send a NMI IPI to
itself is a good idea.

I am debugging a patch based on apic->send_IPI_self(NMI_VECTOR) to replace
"int $2". Something unexpected is happening...

--
regards
Yang, Sheng