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

Sheng Yang
2010-03-18T05:22:28Z

On Thursday 18 March 2010 12:50:58 Zachary Amsden wrote:
> On 03/17/2010 03:19 PM, Sheng Yang wrote:
> > 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.
>
> That's the only defined case, and it is defined because the vector field
> is ignore for DM_NMI. Vol 3A (exact section numbers may vary depending
> on your version).
>
> 8.5.1 / 8.6.1
>
> '100 (NMI) Delivers an NMI interrupt to the target processor or
> processors. The vector information is ignored'
>
> 8.5.2 Valid Interrupt Vectors
>
> 'Local and I/O APICs support 240 of these vectors (in the range of 16 to
> 255) as valid interrupts.'
>
> 8.8.4 Interrupt Acceptance for Fixed Interrupts
>
> '...; vectors 0 through 15 are reserved by the APIC (see also: Section
> 8.5.2, "Valid Interrupt Vectors")'
>
> So I misremembered, apparently you can deliver interrupts 0x10-0x1f, but
> vectors 0x00-0x0f are not valid to send via APIC or I/O APIC.

As you pointed out, NMI is not "Fixed interrupt". If we want to send NMI, it
would need a specific delivery mode rather than vector number.

And if you look at code, if we specific NMI_VECTOR, the delivery mode would be
set to NMI.

So what's wrong here?

--
regards
Yang, Sheng