[PATCH v2 7/7] arm64/sve: Don't zero non-FPSIMD register state on syscall by default

[Mark Brown]

On Wed, Jul 20, 2022 at 10:20:23AM +0100, Will Deacon wrote:
> On Tue, Jul 19, 2022 at 08:35:46PM +0100, Mark Brown wrote:
> > On Tue, Jul 19, 2022 at 06:35:37PM +0100, Catalin Marinas wrote:

> > > > The sysctl is disabled by default since it is anticipated that the risk
> > > > of disruption to userspace is low. As well as being within the
> > > > documented ABI this new behaviour mirrors the standard function call ABI
> > > > for SVE in the AAPCS which should mean that compiler generated code is
> > > > unlikely to rely on the current behaviour, the main risk is from hand
> > > > coded assembly which directly invokes syscalls. The new behaviour is
> > > > also what is currently implemented by qemu user mode emulation.

> > > IIRC both Will and Mark R commented in the past that they'd like the
> > > current de-facto ABI to become the official one. I'll let them comment.

> > That would be good.  I've not heard anything from Will either directly
> > or indirectly.  Mark R has indicated privately directly to me that he
> > originally pushed for the currently implemented behaviour and prefers
> > it.  Marc Zyngier has previously noted publicly the current behaviour
> > being a consideration in the context of discusion of optimisation ideas
> > like this one, I was a bit surprised that he commented on an earlier
> > patch in the series but not this one.  You have previously pushed back
> > on an attempt to document the current ABI, that was the main motivation
> > for writing this patch.

> I remember discussing this somewhere at some point with somebody, but that's
> not a tonne of use!

> My opinion is that the observable behaviour is what matters. So if we
> documented that the register state is "undefined" but it is in fact always
> zero, then we should fix the documentation.

It's always zero with Linux, never zeroed by qemu user.

> Does that help?

Yes, that's clear thanks though it does differ from what Catalin has
said about keeping the flexibility in the documentation - Catalin?
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 488 bytes
Desc: not available
URL: <http://lists.infradead.org/pipermail/linux-arm-kernel/attachments/20220720/93d56651/attachment.sig>