On Mon, Nov 22, 2021 at 06:10:25PM +0000, Marc Zyngier wrote: > Mark Brown <broonie@xxxxxxxxxx> wrote: > > While we're on the subject of potential future work we might in future > > want to not disable SVE on every syscall if (as seems likely) it turns > > out that that's more performant for small vector lengths > How are you going to retrofit that into userspace? This would be an > ABI change, and I'm not sure how you'd want to deal with that > transition... We don't need to change the ABI, the ABI just says we zero the registers that aren't shared with FPSIMD. Instead of doing that on taking a SVE access trap to reenable SVE after having disabled TIF_SVE we could do that during the syscall, userspace can't tell the difference other than via the different formats we use to report the SVE register set via ptrace if it single steps over a syscall. Even then I'm struggling to think of a scenario where userspace would be relying on that. You could also implement a similar optimisation by forcing on TIF_SVE whenever we return to userspace but that would create a cost for userspace tasks that don't use SVE on SVE capable hardware so doesn't seem as good. In any case it's not an issue for now since anything here will need benchmarking on a reasonable range of hardware.
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