On Tue, Nov 23, 2021 at 10:11:33AM +0000, Marc Zyngier wrote:

Mark Brown [off-list ref] wrote:

quoted

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'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?

My point is that this doesn't need to change.  Userspace can't tell if
we zeroed the non-shared state on syscall or on some later access trap.