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

Sheng Yang
2010-03-18T01:19:48Z

On Thursday 18 March 2010 05:14:52 Zachary Amsden wrote:
> On 03/16/2010 11:28 PM, Sheng Yang wrote:
> > 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...
>
> You can't use the APIC to send vectors 0x00-0x1f, or at least, aren't
> supposed to be able to.

Um? Why?

Especially kernel is already using it to deliver NMI.

--
regards
Yang, Sheng