linux-kernel - Re: [PATCH V4 1/2] perf ignore LBR and extra

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Message-ID: <20140709145809.GG9918@twins.programming.kicks-ass.net>
Date:	Wed, 9 Jul 2014 16:58:09 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Liang, Kan" <kan.liang@...el.com>
Cc:	"andi@...stfloor.org" <andi@...stfloor.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.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@...el.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.

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