[PATCH v3 0/8] arm64/sve: First steps towards optimizing syscalls

[Mark Brown]

On Wed, Jul 15, 2020 at 05:49:34PM +0100, Dave Martin wrote:

> I wonder whether we ought to accompany this with a crude mechanism to
> choose dynamically between setting TIF_SVE_NEEDS_FLUSH and keeping the
> old behaviour.

> My concern with doing this unconditionally has been that we can end up
> with TIF_SVE permanently stuck on, which increases the per-task overhead.
> This is not a worry if the user task really does use SVE once per
> context switch, but not so good if, say, the libc startup probes for
> SVE to initialise some internal logic but the task otherwise doesn't
> use it.  (This is just a worry: I haven't looked for evidence to support
> it.)

Yes, it's a concern.  My thought was to follow this up with something
which copies what some of the other architectures are doing for FP
registers and go back to the existing behaviour after a certain number
of syscalls.  That has a bunch of room for debate and bikeshedding about
what exactly an appropriate number might be of course.
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