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. > + /* > + * 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?