[PATCH] arm64: Document boot requirements for FEAT_SME_FA64

[PATCH] arm64: Document boot requirements for FEAT_SME_FA64

[Mark Brown]

The EAC1 release of the SME specification adds the FA64 feature which
requires enablement at higher ELs before lower ELs can use it. Document
what we require from higher ELs in our boot requirements.

Signed-off-by: Mark Brown <broonie@kernel.org>
---
 Documentation/arm64/booting.rst | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/Documentation/arm64/booting.rst b/Documentation/arm64/booting.rst
index 3f9d86557c5e..0047d3b42d5c 100644
--- a/Documentation/arm64/booting.rst
+++ b/Documentation/arm64/booting.rst
@@ -340,6 +340,16 @@ Before jumping into the kernel, the following conditions must be met:
     - SMCR_EL2.LEN must be initialised to the same value for all CPUs the
       kernel will execute on.
 
+  For CPUs with the Scalable Matrix Extension FA64 feature (FEAT_SME_FA64)
+
+  - If EL3 is present:
+
+    - SMCR_EL3.FA64 (bit 31) must be initialised to to 0b1.
+
+ - If the kernel is entered at EL1 and EL2 is present:
+
+    - SMCR_EL2.FA64 (bit 31) must be initialised to to 0b1.
+
 The requirements described above for CPU mode, caches, MMUs, architected
 timers, coherency and system registers apply to all CPUs.  All CPUs must
 enter the kernel in the same exception level.  Where the values documented
-- 
2.30.2


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Re: [PATCH] arm64: Document boot requirements for FEAT_SME_FA64

[Will Deacon]

On Mon, Oct 25, 2021 at 01:48:22PM +0100, Mark Brown wrote:
> The EAC1 release of the SME specification adds the FA64 feature which
> requires enablement at higher ELs before lower ELs can use it. Document
> what we require from higher ELs in our boot requirements.
> 
> Signed-off-by: Mark Brown <broonie@kernel.org>
> ---
>  Documentation/arm64/booting.rst | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/Documentation/arm64/booting.rst b/Documentation/arm64/booting.rst
> index 3f9d86557c5e..0047d3b42d5c 100644
> --- a/Documentation/arm64/booting.rst
> +++ b/Documentation/arm64/booting.rst
> @@ -340,6 +340,16 @@ Before jumping into the kernel, the following conditions must be met:
>      - SMCR_EL2.LEN must be initialised to the same value for all CPUs the
>        kernel will execute on.
>  
> +  For CPUs with the Scalable Matrix Extension FA64 feature (FEAT_SME_FA64)
> +
> +  - If EL3 is present:
> +
> +    - SMCR_EL3.FA64 (bit 31) must be initialised to to 0b1.
> +
> + - If the kernel is entered at EL1 and EL2 is present:
> +
> +    - SMCR_EL2.FA64 (bit 31) must be initialised to to 0b1.

"to to"

With the second bullet, does that mean KVM is in violation of our own
documentation?

Will

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Re: [PATCH] arm64: Document boot requirements for FEAT_SME_FA64

[Mark Brown]

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On Tue, Oct 26, 2021 at 10:29:25AM +0100, Will Deacon wrote:
> On Mon, Oct 25, 2021 at 01:48:22PM +0100, Mark Brown wrote:

> > + - If the kernel is entered at EL1 and EL2 is present:
> > +
> > +    - SMCR_EL2.FA64 (bit 31) must be initialised to to 0b1.

> With the second bullet, does that mean KVM is in violation of our own
> documentation?

No worse than it was before - we already document the requirements for
baseline SME which we don't ourselves satisfy, no guest will be able to
get as far as noticing that FA64 isn't enabled since all SME operations
will trap.

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