On Wed, Feb 07, 2024 at 01:09:51PM +0000, Mark Brown wrote:

On Wed, Feb 07, 2024 at 12:23:56PM +0000, Dave Martin wrote:

quoted

On Mon, Feb 05, 2024 at 05:41:47PM +0000, Mark Brown wrote:

quoted

On Mon, Feb 05, 2024 at 05:11:59PM +0000, Dave Martin wrote:

quoted

quoted

quoted

If the kernel is now juggling two #defines for the maximum vector size,
this feels like it may seed bitrot...

quoted

quoted

Ideally we'd just not have the existing define externally but it's there
and it's been used.

quoted

To clarify, is this intended as a temporary band-aid against silly
behaviour while a cleaner solution is found, or a permanent limitation?

Ideally we'd just make everything dynamic, other than the regset issue
and the bitmasks used for VL enumeration we're there already.  Making
the bitmasks dynamically sized is more painful but are also doing
enumeration that userspace doesn't need to do.

For the bitmasks, we'd be saving (512 - 16) / 8 = 62 bytes for each of
SVE and SME (I think).

The tradeoff really didn't seem worth it...

quoted

We'd need to change various things if the architectural max VL actually
grew, so no forward-portability is lost immediately if the kernel
adopts 16 internally, but I'm still a little concerned that people may
poke about in the kernel code as a reference and this will muddy the
waters regarding how to do the right thing in userspace (I know people
shouldn't, but...)

I think if we fix the ptrace regset issue we're doing a good enough job
of just using fully dynamic sizing with no limits other than what's been
enumerated there.  We could possibly deal with the enumberation code by
changing it to use ZCR/SMCR_ELx_LEN_ based defines so that it's
obviously coming from what we can possibly write to the register but
it's a bit less clear how to do that neatly.

OK, but we still seem to have two competing approaches: clamp SVE_VQ_MAX
for kernel internal purposes, or restore the dynamic sizing of
NT_ARM_SVE based on the new regset core behaviour.

Are you saying we should or both, or otherwise which one?

Cheers
---Dave

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