linux-kernel - Re: [PATCH 4/6] arm64: pmu: Add hook to handle pmu-related undefined instructions

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Message-ID: <20190517082655.GK2623@hirez.programming.kicks-ass.net>
Date:   Fri, 17 May 2019 10:26:55 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Raphael Gault <raphael.gault@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        mingo@...hat.com, catalin.marinas@....com, will.deacon@....com,
        acme@...nel.org
Subject: Re: [PATCH 4/6] arm64: pmu: Add hook to handle pmu-related undefined
 instructions

On Fri, May 17, 2019 at 09:04:20AM +0100, Mark Rutland wrote:

> Remember that this is in an undefined (trap) handler.
> 
> If userspace _attempts_ to write to the registers, the CPU will trap to the
> kernel. The comment is perhaps misleading; when we "do nothing", the common
> trap handling code will send a SIGILL to userspace.
> 
> It would probably be better to say something like:
> 
> 	/*
> 	 * If userspace is tries to read a counter that doesn't exist on this
> 	 * CPU, we emulate it as reading as zero. This happens if userspace is
> 	 * preempted between reading the idx and actually reading the counter,
> 	 * and the seqlock and idx have already changed, so it's as-if the
> 	 * counter has been reprogrammed with a different event.

Might be good to mention that userspace will/should discard the value it
reads, and therefore any value is good (including 0).

> 	 * We don't permit userspace to write to these registers, and will
> 	 * inject a SIGILL.
> 	 */
> 
> There is one caveat: userspace can write to PMSELR without trapping, so we will
> have to context-switch with the task. That only affects indirect addressing of
> PMU registers, and doesn't have a functional effect on the behaviour of the
> PMU, so that's benign from the PoV of perf.

Sad though; ideally you'd state that indirect addressing is
out-of-bounds and they get to keep the pieces. But I suspect you're
right that people will do it anyway and complain once it comes apart.