Re: [PATCH v2 1/3] KVM: arm64: Make ID_AA64MMFR1_EL1.HCX writable from userspace

[Marc Zyngier <maz@kernel.org>]

On Tue, 09 Sep 2025 08:07:15 +0100,
Oliver Upton <oliver.upton@linux.dev> wrote:
> 
> On Tue, Sep 09, 2025 at 11:44:13AM +0800, Jinqian Yang wrote:
> > Allow userspace to downgrade HCX in ID_AA64MMFR1_EL1. Userspace can
> > only change this value from high to low.
> > 
> > Signed-off-by: Jinqian Yang <yangjinqian1@huawei.com>
> 
> I'm not sure our quality of emulation is that great in this case. We
> have no way of trapping the register and it is always stateful. Better
> yet, our RESx infrastructure doesn't account for the presence of
> FEAT_HCX and we happily merge the contents with the host's HCRX.

Yeah, that's not good, and definitely deserves a fix.

> We should make a reasonable attempt at upholding the architecture before
> allowing userspace to de-feature FEAT_HCX.

My concern here is the transitive implications of FEAT_HCX being
disabled: a quick look shows about 20 features that depend on
FEAT_HCX, and we don't really track this. I can probably generate the
dependency graph, but that's not going to be small. Or very useful.

However, we should be able to let FEAT_HCX being disabled without
problem if the downgrading is limited to non-EL2 VMs. Same thing for
FEAT_VHE.

What do you think?

	M.

-- 
Without deviation from the norm, progress is not possible.