[PATCH] arm64/sve: Document that __SVE_VQ_MAX is much larger than needed

[Dave Martin]

On Tue, Feb 06, 2024 at 04:27:01PM +0000, Mark Brown wrote:
> __SVE_VQ_MAX is defined without comment as 512 but the actual
> architectural maximum is 16, a substantial difference which might not
> be obvious to readers especially given the several different units used
> for specifying vector sizes in various contexts and the fact that it's
> often used via macros.  In an effort to minimise surprises for users who
> might assume the value is the architectural maximum and use it to do
> things like size allocations add a comment noting the difference.

Well, the value 512 was semi-deliberately chosen to be surprising.

But the point about units is valid: to the casual reader, "512" does
look suspiciously like a bit count when it really really isn't...

> 
> Signed-off-by: Mark Brown <broonie at kernel.org>
> ---
>  arch/arm64/include/uapi/asm/sve_context.h | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/arm64/include/uapi/asm/sve_context.h b/arch/arm64/include/uapi/asm/sve_context.h
> index 754ab751b523..59f283f373a6 100644
> --- a/arch/arm64/include/uapi/asm/sve_context.h
> +++ b/arch/arm64/include/uapi/asm/sve_context.h
> @@ -13,6 +13,10 @@
>  
>  #define __SVE_VQ_BYTES		16	/* number of bytes per quadword */
>  
> +/*
> + * Note that for future proofing __SVE_VQ_MAX is defined much larger
> + * than the actual architecture maximum of 16.
> + */

I think that putting shadow #defines in comments in UAPI headers is a
really bad idea...  is this a normative statement about the user API,
or what?

My concern is that if we muddy the waters here different bits of
software will do different things and we will get a mess with no
advantages.

Portability issues may ensue if userspace software feels it can
substitute some other value for this constant, since we can't control
what userspace uses it for.

>  #define __SVE_VQ_MIN		1

Would it be sufficient to say something like:

/*
 * Yes, this is 512 QUADWORDS.
 * Never allocate memory or size structures based on the value of this
 * constant.
 */
>  #define __SVE_VQ_MAX		512

Though comments might be better placed alongsize SVE_VQ_MAX at al., in
ptrace.h and sigcontext.h rather than here.  The leading __ should at
least be a hint that __SVE_VQ_MAX shouldn't be used directly by
anyone...

Cheers
---Dave