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

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Date:	Tue, 16 Feb 2016 09:00:15 +0100
From:	Jan Glauber <jan.glauber@...iumnetworks.com>
To:	Will Deacon <will.deacon@....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 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?

Jan

> Will