RE: [PATCH V4 1/2] perf ignore LBR and extra_regs.

[Liang, Kan]

> -----Original Message-----
> From: Peter Zijlstra [mailto:peterz@xxxxxxxxxxxxx]
> Sent: Wednesday, July 09, 2014 10:58 AM
> To: Liang, Kan
> Cc: andi@xxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH V4 1/2] perf ignore LBR and extra_regs.
> 
> On Wed, Jul 09, 2014 at 02:32:28PM +0000, Liang, Kan wrote:
> >
> >
> > > On Tue, Jul 08, 2014 at 09:49:40AM -0700, kan.liang@xxxxxxxxx wrote:
> > > > +/*
> > > > + * Under certain circumstances, access certain MSR may cause #GP.
> > > > + * The function tests if the input MSR can be safely accessed.
> > > > + */
> > > > +static inline bool check_msr(unsigned long msr) {
> > > > +	u64 value;
> > > > +
> > > > +	if (rdmsrl_safe(msr, &value) < 0)
> > > > +		return false;
> > > > +	if (wrmsrl_safe(msr, value) < 0)
> > > > +		return false;
> > > > +	return true;
> > > > +}
> > >
> > > What does this thing return after patch 2? does the write still
> > > fault or will KVM silently take writes too?
> >
> > If applying patch 2, the function will return true. The KVM just simply ignore
> the reads/writes.
> 
> OK, then that's broken too. We want a function to return false for any
> malfunctioning MSR, ignoring writes and returning 0s is not proper
> functioning.

The patch 2 is to handle the case that the administrator can only patch the host. Don't need to force user to upgrade their guest to fix the crash.  
And ignore the annoying "unhandled...." KVM messages

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