Re: perf with precise attribute kills all KVM based VMs

[David Ahern <dsahern@xxxxxxxxx>]

On 7/9/12 8:39 AM, Avi Kivity wrote:
> On 07/09/2012 05:24 PM, Peter Zijlstra wrote:
> > On Mon, 2012-07-09 at 17:19 +0300, Gleb Natapov wrote:
> > > Yes, this is knows problem that I can't find time to fix. The crash is
> > > cause by CPU using host PEBS virtual address while guest is running
> > > which causes guest memory corruption. We should disable evens that use
> > > PEBS at the guest entry.
> >
> > Whoops.. so the hardware is reading the DS address as programmed by the
> > host (host linear address) and using it as a guest linear address?
> >
> > Quality stuff..
> >
> > Disabling PEBS events for guests isn't pretty though..
>
> We already have atomic MSR switching at guest entry/exit time.  So it's
> not pretty in terms of not getting full profiling, but the code won't be
> too hard.  Basically we just have to exclude_guest any pebs event.

I found this testing changes to perf-kvm, but found the problem extends 
to just perf-record. With perf-record exclude_guest defaults to 1. See 
tools/perf/util/util.c, event_attr_init().

> > but I guess the
> > only alternative is mapping the DS into the guest and reprogramming
> > MSR_IA32_DS_AREA which is all a little involved I suppose?
>
> Way too involved, especially as it's virtual addresses and we don't
> control the guest cr3.  Note that the hardware won't fail gracefully.


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