Re: [PATCH 00/11] KVM: arm64: nv: Align feature limitations with current state of support

[Marc Zyngier <maz@kernel.org>]

On Fri, 12 Sep 2025 22:22:47 +0100,
Oliver Upton <oliver.upton@linux.dev> wrote:
> 
> There's a mix of features we currently hide for NV VMs that we actually
> can already support, either because they have no impact on our NV
> implementation or the feature was already implemented and someone forgot
> to update the mask...
> 
> On top of that, I decided to invert our masking for several registers to
> have denylist behavior. This is a better way to express things, IMO, as
> the features we do not support are explicitly listed.

This was done for a reason: as long as NV was out of tree, it was far
easier to pick what we wanted to support, rather than playing
wack-a-mole as new features were popping up in the non-NV code.

Now that NV is in, and that NV support is part of the basic
requirements for anything that we add to KVM, it is probably the right
time to invert the logic.

> In so doing, I found a bug relating to FEAT_DoubleLock where we
> accidentally claim support where we shouldn't as the 0 value implies
> the feature is implemented.

Meh. Don't you love negative features?

> I've made each relaxation into its own patch to capture the rationale in
> the changelog but I'm also fine squashing these based on the affected
> register. Features that limit our MMU implementation are deliberately
> left masked, e.g. FEAT_ETS* and FEAT_nTLBPA place constraints on the
> behavior of our nested MMU. Although the latter is highly unlikely to
> matter.

Well, unlikely to matter for S2. For S1, this is a different story.

> Applies to 6.17-rc4. **NOTE** this depends on Jinqian's series to make
> ID_AA64MMFR1_EL1.TWED writable [*], that should be applied before this
> series.

Thanks for that. I'll have a look shortly.

	M.

-- 
Jazz isn't dead. It just smells funny.