[PATCH] crypto: arm64/ghash - Fix incorrect output from ghash-neon

[Ard Biesheuvel]

On Mon, 15 Dec 2025 at 21:16, Eric Biggers <ebiggers at kernel.org> wrote:
>
> On Mon, Dec 15, 2025 at 04:54:34PM +0900, Ard Biesheuvel wrote:
> > > > All 64-bit RPi models except the RPi5 are affected by this, as those
> > > > do not implement the crypto extensions. So I would expect QEMU to do
> > > > the same.
> > > >
> > > > It would be nice, though, if we could emulate this on the mach-virt
> > > > machine model too. It should be fairly trivial to do, so if there is
> > > > demand for this I can look into it.
> > >
> > > I'm definitely interested in it.  I'm already testing multiple "-cpu"
> > > options, and it's easy to add more.
> > >
> > > With qemu-system-aarch64 I'm currently only using "-M virt", since the
> > > other machine models I've tried don't boot with arm64 defconfig,
> > > including "-M raspi3b" and "-M raspi4b".
> > >
> > > There may be some tricks I'm missing.  Regardless, expanding the
> > > selection of available CPUs for "-M virt" would be helpful.  Either by
> > > adding "real" CPUs that have "interesting" combinations of features, or
> > > by just allowing turning features off like
> > > "-cpu max,aes=off,pmull=off,sha256=off".  (Certain features like sve can
> > > already be turned off in that way, but not the ones relevant to us.)
> > >
> >
> > There are some architectural rules around which combinations of crypto
> > extensions are permitted:
> > - PMULL implies AES, and there is no way for the ID registers to
> > describe a CPU that has PMULL but not AES
> > - SHA256 implies SHA1 (but the ID register fields are independent)
> > - SHA3 and SHA512 both imply SHA256+SHA1
> > - SVE versions are not allowed to be implemented unless the plain NEON
> > version is implemented as well
> > -  FEAT_Crypto has different meanings for v8.0, v8.2 and v9.x
> >
> > So it would be much easier, also in terms of future maintenance, to
> > have a simple 'crypto=off' setting that applies to all emulated CPU
> > models, given that disabling all crypto on any given compliant CPU
> > will never result in something that the architecture does not permit.
> >
> > Would that work for you?
>
> I thought it had been established that the "crypto" grouping of features
> (as implemented by gcc and clang) doesn't reflect the actual hardware
> feature fields and is misleading because additional crypto extensions
> continue to be added.
>
> I'm not sure that applies here, but just something to consider.
>

You are right, this is why 'crypto=on' can never mean anything other
than 'do not disable the crypto extensions that this particular CPU
type provides' But that does not mean 'crypto=off' is equally
problematic.

> There's certainly no need to support emulating combinations of features
> that no hardware actually implements.  So yes, if that means "crypto" is
> the right choice, that sounds fine.
>

OK, I'll have a stab at that and cc you on the patches.