Re: [RFT] mmu optimizations branch

[Ingo Molnar <mingo-X9Un+BFzKDI@public.gmane.org>]

* Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org> wrote:

> >unfortunately 0xc011f7f3 is in native_write_msr(), which isnt very 
> >helpful. (i have CONFIG_PARAVIRT enabled in the -rt guest and host 
> >kernels) But the MSR values suggest that this is the NMI watchdog 
> >thing again, trying to program MSR_ARCH_PERFMON_EVENTSEL0 and 
> >MSR_ARCH_PERFMON_PERFCTR0, but this time Linux recovered due to a 
> >more robust MSR handling. The guest disabled the NMI watchdog with:
> >
> >  Testing NMI watchdog ... CPU#0: NMI appears to be stuck (0->0)!
> >
> >the FC6 installer hang that i saw with earlier MMU-branch snapshots 
> >is fixed.
> 
> 
> Good.  Handling the counter well would have been very difficult, 
> especially if attempting to support cross migration.

as far as the NMI watchdog goes, it's in fact better to keep it disabled 
this way - it's not like the guest context could 'lock up' in an 
undebuggable way. Any NMI activity in the guest context would be pretty 
pointless. I'd suggest simulating a non-working performance counter: 
i.e. dont inject a #GPF when doing the wrmsr, and maybe preserve the 
values that were written into the MSR register, but otherwise dont try 
to implement the functionality by injecting NMIs. Worst-case this could 
result in user-space debugging tools seeing non-working 
performance-counter functionality.

	Ingo

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