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

[Mark Brown]

On Tue, May 26, 2026 at 01:03:19PM +0100, Mark Rutland wrote:
> On Tue, May 26, 2026 at 11:19:38AM +0100, Mark Brown wrote:

> The part I asked for clarification on was:

>   AFAICT, none of the values for the SMCR_ELx.LEN and ZCR_ELx.LEN fields
>   are reserved or unallocated. Thus all the bits of those fields should
>   be stateful, and a read should observe the last value written,
>   regardless of the effective value of the field.

>   [...]

>   Either what we're doing is wrong, or the architcture requires a
>   clarification to say that values corresponding to unimplmented vector
>   lengths are reserved.

...

> While one simple reading is that all values must be preserved exactly, I
> don't think this watertight, and I think while some people will take
> reading (a), others will take (b) or (c). I should have spelled that out
> more clearly in my initial mail, sorry.

I see.  Given the general tendency to brevity it didn't occur to me that
options b or c might be a thing.

FWIW the restriction to powers of 2 was a revision in the architecture,
originally all values were allowed, though it's there now so the history
doesn't make a huge difference.

Attachment: signature.asc
Description: PGP signature