On Mon, 22 Nov 2021 18:30:16 +0000, Mark Brown <broonie@xxxxxxxxxx> wrote: > > 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. That's not the point I'm trying to make. Userspace expects to have lost SVE information over a syscall (even if the VL is 128, it expects to have lost P0..P15 and FFR). How do you plan to tell userspace that this behaviour has changed? M. -- Without deviation from the norm, progress is not possible.
