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

[Sheng Yang <sheng@linux.intel.com>]

On Wednesday 17 March 2010 17:41:58 Avi Kivity wrote:
> On 03/17/2010 11:28 AM, Sheng Yang wrote:
> >> 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.
> 
> That's pretty bad, as NMI runs on a separate stack (via IST).  So if
> another NMI happens while our int $2 is running, the stack will be
> corrupted.

Though hardware didn't provide this kind of block, software at least would 
warn about it... nmi_enter() still would be executed by "int $2", and result 
in BUG() if we are already in NMI context(OK, it is a little better than 
mysterious crash due to corrupted stack).
> 
> > 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...
> 
> I think you need DM_NMI for that to work correctly.
> 
> An alternative is to call the NMI handler directly.

apic_send_IPI_self() already took care of APIC_DM_NMI.

And NMI handler would block the following NMI?

-- 
regards
Yang, Sheng