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

[Peter Zijlstra]

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.

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