Re: [PATCH v1 1/3] KVM: arm64: nv: Don't advertise FEAT_LPA2 for guest stage-1

[Marc Zyngier <maz@kernel.org>]

On Thu, 10 Sep 2026 11:56:05 +0100,
Wei-Lin Chang <weilin.chang@arm.com> wrote:
> 
> On Wed, Sep 09, 2026 at 10:44:58PM +0000, sashiko-bot@kernel.org wrote:
> > Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> > - [Medium] Equality checks for TGRAN4 and TGRAN16 fail to mask future architectural enum values, leaking unsupported hardware capabilities to NV guests.
> > --
> > 
> > commit 6c2ebbb5542b3e22eaee90b7cf8d8cbbc9704e7c
> > Author: Wei-Lin Chang <weilin.chang@arm.com>
> > 
> > KVM: arm64: nv: Don't advertise FEAT_LPA2 for guest stage-1
> > 
> > There is no FEAT_LPA2 support for NV guests as of now, but we missed
> > limiting it for the non-stage-2 granule size fields (TGRAN4, TGRAN16) in
> > ID_AA64MMFR0_EL1. Add the sanitisation.
> > 
> > > diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
> > > index 17123f0b6daba..4d1884b32fc48 100644
> > > --- a/arch/arm64/kvm/nested.c
> > > +++ b/arch/arm64/kvm/nested.c
> > > @@ -1731,6 +1731,19 @@ u64 limit_nv_id_reg(struct kvm *kvm, u32 reg, u64 val)
> > >  		/* Hide CNTPOFF if present */
> > >  		val = ID_REG_LIMIT_FIELD_ENUM(val, ID_AA64MMFR0_EL1, ECV, IMP);
> > >  
> > > +		/* NV does not support FEAT_LPA2. */
> > > +		if (SYS_FIELD_GET(ID_AA64MMFR0_EL1, TGRAN4, val) ==
> > > +		    ID_AA64MMFR0_EL1_TGRAN4_52_BIT) {
> > 
> > [Severity: Medium]
> > Will this exact equality check fail to mask future architectural enum values?
> > 
> > If future hardware implements a capability enum value greater than 52_BIT for
> > TGRAN4, this check will evaluate to false, leaking the unsupported capability
> > to NV guests.
> > 
> > Could this be capped instead, perhaps using inequalities or the
> > ID_REG_LIMIT_FIELD_ENUM macro, to maintain forward compatibility?
> 
> ID_REG_LIMIT_FIELD_ENUM works only for unsigned fields, perhaps it is
> worth extending it so it can deal with signed enums too.

Don't bother. We're not doing "crystal ball architecture".

	M.

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