On Fri, Aug 11, 2023 at 04:32:10PM +0100, Catalin Marinas wrote:

On Mon, Aug 07, 2023 at 11:00:23PM +0100, Mark Brown wrote:

quoted

+		gcs_free(current);
+		current->thread.gcs_el0_mode = 0;
+		write_sysreg_s(0, SYS_GCSCRE0_EL1);
+		write_sysreg_s(0, SYS_GCSPR_EL0);
+	}
+}

Do we need and isb() or there's one on this path? If it's only EL0
making use of this register, we should be fine with the ERET before
returning to user. Not sure whether the kernel uses this, GCSSTTR
doesn't need it.

They're only used by EL0, at EL1 we do read GCSPR for signal handling
but AIUI that shouldn't be any more of an issue than it is for the
TPIDRs which we don't have a barrier for.  It's possible I'm
misunderstanding though.

quoted

+	/*
+	 * 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();

What's this barrier for? The spec (at least the version I have) only
talks about accesses, nothing to do with the registers that we context
switch here.

Right, it's for the GCS memory rather than the registers.  I'm fairly
sure it's excessive but but was erring on the side of caution until I
have convinced myself that the interactions between GCS barriers and
regular barriers were doing the right thing, until we have physical
implementations to contend with I'd guess the practical impact will be
minimal.