[PATCH v1 12/38] arm64/sme: Provide ABI documentation for SME

[Mark Brown]

On Fri, Oct 08, 2021 at 04:45:44PM +0000, Alan Hayward wrote:
> > On 8 Oct 2021, at 16:28, Mark Brown <broonie at kernel.org> wrote:
> > On Fri, Oct 08, 2021 at 02:11:46PM +0000, Alan Hayward wrote:

> >> Can NT_ARM_SSVE return a fpsimd?

> > It's documented that way for simplicity but in the current
> > implementation it won't ever actually do so in practice.  The
> > only case where I could see that it might happen would be if we
> > change the syscalls to stay in streaming mode over syscall, in
> > that case we could do as we do for SVE and preserve FPSIMD
> > registers only.  At present we drop out of streaming mode if we
> > get a syscall with it enabled so it's a non-issue, if people
> > agree that that's the right thing for the syscalls then we should
> > update the documentation to specify this since otherwise we'll
> > doubtless catch someone by surprise if we ever manage to start
> > doing it in the future.

> ….or it’ll end up executing some code that was written to cope with fpsimd, but has never been
> run. Might be worth making it explicit in the documentation.

I will if/when it gets fixed that way.  Actually, while looking
at that code I was tempted to remove the support for returning
FPSIMD only registers via NT_ARM_SVE entirely and just always
convert to SVE format - I'm not sure what the use case was there?
It's not a pressing thing but it seemed like it was a bit of an
implementation detail that we were revealing.
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