Re: [PATCH v10 19/40] arm64/gcs: Context switch GCS state for EL0

Catalin Marinas
2024-08-21T08:50:30Z

On Tue, Aug 20, 2024 at 06:56:19PM +0100, Mark Brown wrote:
> On Mon, Aug 19, 2024 at 04:44:52PM +0100, Mark Brown wrote:
> > On Mon, Aug 19, 2024 at 12:46:13PM +0100, Catalin Marinas wrote:
> > > On Thu, Aug 01, 2024 at 01:06:46PM +0100, Mark Brown wrote:
>
> > > > + /*
> > > > + * Ensure that GCS changes are observable by/from other PEs in
> > > > + * case of migration.
> > > > + */
> > > > + if (task_gcs_el0_enabled(current) || task_gcs_el0_enabled(next))
> > > > + gcsb_dsync();
>
> > > Could we do the sysreg writing under this 'if' block? If no app is using
> > > GCS (which would be the case for a while), it looks like unnecessary
> > > sysreg accesses.
>
> > Yes, that should be fine I think.
>
> I forgot when writing the above that we always allow reads from
> GCSPR_EL0 in order to avoid corner cases for unwinders in the case of
> asynchronous disable. I'd expect that to be cheap to access though.

But then gcs_preserve_current_state() doesn't save the GCSPR_EL0 value
if the shadow stack was disabled. At the subsequent switch to this task,
we write some stale value.

--
Catalin