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

[Dave Martin <Dave.Martin@arm.com>]

On Wed, Feb 07, 2024 at 12:48:58PM +0000, Mark Brown wrote:
> On Wed, Feb 07, 2024 at 12:01:43PM +0000, Dave Martin wrote:
> > On Tue, Feb 06, 2024 at 04:27:01PM +0000, Mark Brown wrote:
> 
> > > +/*
> > > + * 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?
> 
> Well, the only reason I'm mentioning the constant here is that
> __SVE_VQ_MIN is defined too and has a perfectly good value, things look
> a bit neater with a shared comment block.  I'm not sure there's a hugely
> meaingful difference between having a comment adjacent to a named
> constant in a header and one a couple of lines away that mentions the
> constant by name.

It wasn't so much the exact location that concerned me, rather putting
it in a UAPI header at all.

Maybe so long as the comment doesn't quote a literal value for the arch
max VQ that would be better.  If there is a value there, we may be kind
of legitimising its use even if it's in a comment...

> 
> > 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.
> 
> I don't think we want people using this at all, ideally we'd remove it
> but it's in the uapi.

I think the main legitimate uses are for implementing sve_vl_valid() and
for type selection purposes (analogous to the C <limits.h> constants --
though all the "obvious" types are fine so this is a but redundant).
But yeah, it's there now.

> > 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
> 
> I think the fact that this vector length is more than an order of
> magnitude more than is architecturally supported at present needs to be
> conveyed, it's perfectly reasonable for people to not want to do dynamic
> allocation/sizing of buffers in some applications and the above sounds
> more like stylistic guidance about using dynamic sizing to improve
> memory usage.

I guess that's true; people need to know that they'll be allocating a
silly amount of memory if they use the existing _MAX constants directly.
Laziness is a perfectly good reason for doing this for development hacks
that aren't going to be published, less so for code that ends up in
libraries or otherwise gets into the wild...

I preferred to encourage people to size dynamically, but we don't have
a way to enforce it.

Ideally there would be a direct way to read out the system-wide max VL
to provide userspace with a sensible default allocation size, but that
doesn't really exist today (though ptrace and PR_SVE_{SET,GEL}_VL
provide ways to find out, but it's a bit grungy).

How about something along the lines of:

/*
 * Yes, this is 512 QUADWORDS.
 * To help ensure forward portability, this is much larger than the
 * current maximum value defined by the SVE architecture.
 * While arrays or static allocations can be sized based on this value,
 * watch out!  It will waste a surprisingly large amount of memory.
 * Dynamic sizing based on the actual runtime vector length is likely to
 * be preferable for most purposes.
 */

> 
> > 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...
> 
> Yeah, the multiple layers of indirection aren't helpful here.  We
> probably need to comment it in both places TBH.

Agreed, part of that came from a desire to avoid duplicating
information.  I think the indirection via sve_context.h was introduced
to work around a bad interaction with user C library headers, but I'm a
bit hazy on that now without digging through the history.

Cheers
---Dave