Re: [PATCH 4/4] KVM: arm64: at: Update AF on software walk only if VM has FEAT_HAFDBS

[Marc Zyngier <maz@kernel.org>]

On Fri, 28 Nov 2025 10:09:46 +0000,
Alexandru Elisei <alexandru.elisei@arm.com> wrote:
> 
> A guest can write 1 to TCR_ELx.HA, making the KVM software walker update
> the access flag in a table descriptor even if FEAT_HAFDBS is not present.
> Avoid this by making wi->ha depend on FEAT_HAFDBS being enabled in the VM,
> similar to how the software walker treats FEAT_HPDS.
> 
> This is not needed for VTCR_EL2.HA, since a guest will always write to
> the in-memory copy of the register, where the HA bit is masked (set to
> 0) by KVM if the VM doesn't have FEAT_HAFDBS.
> 
> Fixes: c59ca4b5b0c3 ("KVM: arm64: Implement HW access flag management in stage-1 SW PTW")
> Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
> ---
>  arch/arm64/kvm/at.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/at.c b/arch/arm64/kvm/at.c
> index 6d41a95f6c60..53bf70126f81 100644
> --- a/arch/arm64/kvm/at.c
> +++ b/arch/arm64/kvm/at.c
> @@ -346,7 +346,8 @@ static int setup_s1_walk(struct kvm_vcpu *vcpu, struct s1_walk_info *wi,
>  
>  	wi->baddr &= GENMASK_ULL(wi->max_oa_bits - 1, x);
>  
> -	wi->ha = (wi->regime == TR_EL2 ?
> +	wi->ha  = kvm_has_feat(vcpu->kvm, ID_AA64MMFR1_EL1, HAFDBS, AF);
> +	wi->ha &= (wi->regime == TR_EL2 ?
>  		  FIELD_GET(TCR_EL2_HA, tcr) :
>  		  FIELD_GET(TCR_HA, tcr));

This is yet another case where we should expand the sanitisation
infrastructure to cover the TCR registers.

Thanks,

	M.

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