On Tue, Jan 23, 2024 at 03:44:23PM +0000, Dave Martin wrote:
> On Mon, Jan 22, 2024 at 08:41:51PM +0000, Mark Brown wrote:
> > When we documented that we always clear state not shared with FPSIMD we

> Where / when?

In the document that is being modified when it was written.

> > -* In practice the affected registers/bits will be preserved or will be replaced
> > -  with zeros on return from a syscall, but userspace should not make
> > -  assumptions about this.  The kernel behaviour may vary on a case-by-case
> > -  basis.

> This was originally an intentionally conservative statement, to allow
> the kernel the flexibility to relax the register zeroing behaviour in
> the future.  It would have permitted not always disabling a task's SVE
> across a syscall, for example.  There were some concerns about security
> and testability that meant that we didn't use this flexibility to begin
> with.

> If we are making an irrevocable commitment not to use this flexibility
> ever, then this comment can go, but if we're not totally sure then I
> think it would be harmless to keep it (?)

I think everyone except for Catalin had felt that the original
discussion had concluded that there was a commitment to always clear the
non-shared bits and was disappointed to learn that the documentation
said otherwise.  When I tried to take advantage of this as part of
optimising the system call overhead for SVE there were eventually
complaints.

> (Feel free to point me to the relevant past discussion that I may have
> missed.)

See the discussion on my syscall optimisation series:

    https://lore.kernel.org/all/20220620124158.482039-8-broonie@kernel.org/