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

[Mark Brown]

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.

> What's the GCSB DSYNC supposed to do here? The Arm ARM talks about
> ordering between GCS memory effects and other memory effects. I haven't
> looked at the memory model in detail yet (D11.9.1) but AFAICT it has
> nothing to do with the system registers. We'll need this barrier when
> ordering is needed between explicit or implicit (e.g. BL) GCS accesses
> and the explicit classic memory accesses. Paging comes to mind, so maybe
> flush_dcache_page() would need this barrier. ptrace() is another case if
> the memory accessed is a GCS page. I can see you added it in other
> places, I'll have a look as I go through the rest. But I don't think one
> is needed here.

It's not particuarly for the system registers, is there's so that
anything else that looks at the task's GCS sees the current state.  I'm
pretty confident this excessive, the goal was to err on the side of
correctness and then relax later.
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