Re: [PATCH v2] arm64/fpsimd: Only provide the length to cpufeature for xCR registers

[Mark Brown]

On Thu, Aug 03, 2023 at 05:39:38PM +0100, Catalin Marinas wrote:
> On Mon, Jul 31, 2023 at 02:58:48PM +0100, Mark Brown wrote:

> > Since the only field we are interested in having the cpufeature code
> > handle is the length field and we use a custom read function to obtain
> > the value we can avoid these warnings by filtering out all other bits
> > when we return the register value, if we're doing that we don't need to
> > bother reading the register at all and can simply use the RDVL/RDSVL
> > value we were filling in instead.

> Maybe that's the simplest fix, especially if you want it in stable, but

Yeah, it's definitely the sort of change we want as a fix - anything
more invasive would be inappropriate.

> I wonder why we even bother with with treating ZCR_EL1 and SMCR_EL1 as
> feature registers. We already have verify_sme_features() to check for
> the mismatch. BTW, is vec_verify_vq_map() sufficient so that we can skip
> the maximum vector length check?

Both enumeration mechanisms were added in the initial series supporting
SVE for reasons that are not entirely obvious to me.  The changelogs
explain what we're doing with the pseudo ID register stuff but do not
comment on why.  There is a cross check between the answers the two give
which appears to be geared towards detecting systems with asymmetric
maximum VLs for some reason but I'm not sure why that's done given that
we can't cope if *any* VL in the committed set is missing, not just the
maximum.  It's not KVM because that has even stricter requirements and
can't cope with any extra supported VLs.  I'm not sure if the inclusion
in the cpufeature stuff is supposed to expose the registers via
emulation of mrs, though a brief test appears to indicate that this
doesn't actually work if it was ever intended to.

The whole thing is very suspect but given that we don't currently have
any ability to emulate systems with asymmetric vector lengths I'm a bit
reluctant to poke at it.

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