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

[Mark Rutland <mark.rutland@arm.com>]

On Fri, May 17, 2019 at 09:10:18AM +0200, Peter Zijlstra wrote:
> On Thu, May 16, 2019 at 02:21:46PM +0100, Raphael Gault wrote:
> > In order to prevent the userspace processes which are trying to access
> > the registers from the pmu registers on a big.LITTLE environment we
> > introduce a hook to handle undefined instructions.
> > 
> > The goal here is to prevent the process to be interrupted by a signal
> > when the error is caused by the task being scheduled while accessing
> > a counter, causing the counter access to be invalid. As we are not able
> > to know efficiently the number of counters available physically on both
> > pmu in that context we consider that any faulting access to a counter
> > which is architecturally correct should not cause a SIGILL signal if
> > the permissions are set accordingly.
> 
> The other approach is using rseq for this; with that you can guarantee
> it will never issue the instruction on a wrong CPU.
> 
> That said; emulating the thing isn't horrible either.

Yup. Attempting to use rseq is on the todo list.

> > +	/*
> > +	 * We put 0 in the target register if we
> > +	 * are reading from pmu register. If we are
> > +	 * writing, we do nothing.
> > +	 */
> 
> Wait _what_ ?!? userspace can _WRITE_ to these registers?

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.
	 *
	 * 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.

Thanks,
Mark.