Re: [PATCH] KVM: arm64: Preserve all guest ZCR_EL2.LEN values

[Mark Brown <broonie@kernel.org>]

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On Mon, May 25, 2026 at 07:36:50PM +0100, Mark Rutland wrote:
> On Fri, May 22, 2026 at 07:00:04PM +0100, Mark Brown wrote:

> > Since b3d29a823099 ("KVM: arm64: nv: Handle ZCR_EL2 traps") when guests
> > write to ZCR_EL2 we have clamped the value of ZCR_EL2.LEN to be at most
> > that configuring the maximum guest VL. This is not the behaviour the

...

> Have we sought feedback from architects? While I said "*or* the
> architcture requires a clarification", I think it should be clarified
> more explicitly either way given that the pattern is unusual.

> Given this is particularly subtle, please keep me in the loop when
> speaking with architects about this.

TBH it didn't strike me as subtle, I don't see anything in the
architecture which would lead me to expect the current behaviour.  The
psudocode all just has direct assignments for the write and there's
language in the ARM (eg, in the ZCR_EL2 description) saying "for all
purposes other than returning the result of a direct read of ZCR_EL2"
which seems specifically intended to cover there being a divergence
between the written and effective values, though I guess it doesn't
explicitly mention writes in the text.  

When I saw the code it looked like something KVM was doing for some
KVM reason that was obscure to me, the lack of clarity seemed like it
was on the KVM side.

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