[BOOT-WRAPPER PATCH 3/3] aarch64: Enable use of GCS for EL2 and below

[Mark Brown]

On Tue, Nov 26, 2024 at 06:01:56PM +0000, Mark Rutland wrote:

> GCSCR_EL3.PCRSEL resets to 0, so the HW won't push to the GCS for BL/BLR
> and won't pop from the GCS for a RET. That menas there's no GCS value
> for a return value check to compare with...

Oh, good point - I'd trusted that the initialisations were required and
misremembered which of PCRSEL and RVCHKEN was which.  Sorry about that.
The values on reset follow the same pattern in all the GCS control
registers down to GCSCRE0_EL1 so the initialisations of the other
control registers are equally redundant.  We should be consistent here,
either initialising all the GCS control registers or relying on the
architecture defaults for all of them, and the note in the changelog
about them needs an update if the initialisation is there.

> That does raise the question of what specifically happens for a return
> at EL3 when GCSCR_EL3.{PCRSEL,RVCHKEN} == {1,0}. Can you enlighten me?

RET will attempt to load and use a GCS record, the pseudocode is in
LoadCheckGCSRecord() which isn't EL dependent other than the selection
of which GCSPR and GCSCR to use and setting the access as privileged if
we're not at EL0.
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