Re: [PATCH v12 02/29] arm64/fpsimd: Update FA64 and ZT0 enables when loading SME state

[Mark Brown]

On Sun, Jul 19, 2026 at 09:35:53AM +0100, Marc Zyngier wrote:
> Mark Brown <broonie@xxxxxxxxxx> wrote:

> > I've gone and implemented the first which will suppress the isb() for
> > current host kernel SME usage, the second two are starting to get more
> > fiddly than seems sensible to do right now.

> I really wish you didn't optimise anything at all at this stage.

> "Optimisation" is exactly what got us into so much trouble over the
> past two years, and I really don't want SME in KVM to follow the same
> trajectory.

> So leave this is a straight ISB, no optimisation. Once you come back
> with actual data showing that this is a terrible bottleneck affecting
> real workloads on real HW, we'll look at it. But until then, please
> keep it as stupid as possible.

I've already left the KVM hypervisor code like that - I only updated the
code here in the main kernel since that keeps the end effect for
existing systems and workloads the same.  I am aware that there has been
a great deal of attention paid to the barriers we have in the context
switch path so I am concerned that other people might have a different
view on adding one.

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