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

[Oliver Upton <oliver.upton@linux.dev>]

On Fri, Sep 12, 2025 at 10:43:05PM +0100, Marc Zyngier wrote:
> > 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?

Just about as fun as negative polarity traps.

> > 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.

Indeed -- just my general impression is that we shouldn't constrain
ourselves in terms of future MMU / TLB behavior at this point.

> > 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.

I'll also have a look at the series, need to revisit an open Q from the
last spin.

Thanks,
Oliver