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

[Peter Zijlstra <peterz@infradead.org>]

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On Wed, Jul 09, 2014 at 03:43:45PM +0000, Liang, Kan wrote:
> 
> 
> > -----Original Message-----
> > From: Peter Zijlstra [mailto:peterz@infradead.org]
> > Sent: Wednesday, July 09, 2014 10:58 AM
> > To: Liang, Kan
> > Cc: andi@firstfloor.org; linux-kernel@vger.kernel.org; kvm@vger.kernel.org
> > 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@intel.com 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

Sure; but what I meant was, check_msr() is broken when ran on such a
kernel. You need to fix check_msr() to return failure on these 'ignored'
MSRs, after all they don't function as expected, they're effectively
broken.

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