[PATCH v10 04/40] arm64: Document boot requirements for Guarded Control Stacks

[Mark Brown]

On Thu, Aug 15, 2024 at 06:00:15PM +0100, Catalin Marinas wrote:
> On Thu, Aug 01, 2024 at 01:06:31PM +0100, Mark Brown wrote:

> > +  - If EL2 is present:

> > +    - GCSCR_EL2 must be initialised to 0.

> > + - If the kernel is entered at EL1 and EL2 is present:
> > +
> > +    - GCSCR_EL1 must be initialised to 0.
> > +
> > +    - GCSCRE0_EL1 must be initialised to 0.

> Currently booting.rst doesn't list *_EL1 registers to be initialised
> when the kernel is entered at EL1, that would usually be the
> responsibility of EL1. The exception is some bits in SCTLR_EL1 around
> not entering with the MMU and caches enabled. But here I think it makes
> sense to add these GCS registers since if some random bits are set, they
> can affect kernels (and user apps) that don't have GCS support.

Right, exactly - the trouble here is that if we enter EL1 with GCS
enabled we aren't able to do function calls until we either disable GCS
or configure the MMU and allocate a GCS.  This means that all existing
kernels which haven't heard of GCS require that GCS be disabled prior to
starting, they'll just fault within a couple of instructions whenever
they reach the EL for which GCS is enabled so it seems sensible to just
require that this is set up.  It is hard to envision a scenario in which
it would be reasonable to start in a different configuration.

Now I think about it I should move those two to not depend on EL2 being
present, that's just cut'n'paste.

> Don't we need HCRX_EL2.GCSEn to be set when entered at EL1?

Yes, if we want GCS to do anything.  I've added this.
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