Re: [PATCH v2 2/5] KVM: arm64: Get rid of host SVE tracking/saving

On Mon, 22 Nov 2021 18:30:16 +0000,
Mark Brown [off-list ref] wrote:

On Mon, Nov 22, 2021 at 06:10:25PM +0000, Marc Zyngier wrote:

quoted

Mark Brown [off-list ref] wrote:

quoted

quoted

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

quoted

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.

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