linux-kernel - Re: [PATCH v3 5/5] arm64/perf: Extend event mask for ARMv8.1

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Date:	Tue, 16 Feb 2016 15:12:53 +0000
From:	Will Deacon <will.deacon@....com>
To:	Jan Glauber <jan.glauber@...iumnetworks.com>
Cc:	Mark Rutland <mark.rutland@....com>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3 5/5] arm64/perf: Extend event mask for ARMv8.1

On Tue, Feb 16, 2016 at 09:00:15AM +0100, Jan Glauber wrote:
> On Mon, Feb 15, 2016 at 08:04:04PM +0000, Will Deacon wrote:
> 
> [...]
> 
> > On Wed, Feb 03, 2016 at 06:12:00PM +0100, Jan Glauber wrote:
> > > +		cpu_pmu->event_mask = 0xffff;	/* ARMv8.1 extended events */
> > > +	else
> > > +		cpu_pmu->event_mask = ARMV8_EVTYPE_EVENT;
> > 
> > ... although can't we just update ARMV8_EVTYPE_EVENT to be 0xffff now?
> > AFAICT, that just eats into bits that used to be RES0, so we shouldn't
> > see any problems. That should make your patch *much* simpler!
> 
> That would of course be easier, but I just can't assess the implications.
> 
> Probably I'm missing something but to me it looks like the event mask is the
> only verification we do for the user-space selectable events. Is it safe for
> implementations that only support 0x3ff events to allow access to the
> whole 0xffff range? What memory would be accessed for non-existing
> events?

Which memory? The worst-case is that we end up writing to some bits in
a register (e.g. PMXEVTYPER) that are RES0 in ARMv8 afaict.

Will