On Thu, Aug 01, 2024 at 01:06:46PM +0100, Mark Brown wrote:
diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c index 4ae31b7af6c3..5f00cb0da9c3 100644 --- a/arch/arm64/kernel/process.c +++ b/arch/arm64/kernel/process.c
[...]
+static void gcs_thread_switch(struct task_struct *next) +{
- if (!system_supports_gcs())
return;- gcs_preserve_current_state();
- gcs_set_el0_mode(next);
- write_sysreg_s(next->thread.gcspr_el0, SYS_GCSPR_EL0);
- /*
* 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.
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.