[PATCH v2 02/14] KVM: arm64: Hide ID_AA64MMFR2_EL1.NV from guest and userspace

[Sebastian Ott]

Hi Marc,

On Thu, 20 Feb 2025, Marc Zyngier wrote:
> Since our take on FEAT_NV is to only support FEAT_NV2, we should
> never expose ID_AA64MMFR2_EL1.NV to a guest nor userspace.
>
> Make sure we mask this field for good.
>
> Signed-off-by: Marc Zyngier <maz at kernel.org>
> ---
> arch/arm64/kvm/sys_regs.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 82430c1e1dd02..9f10dbd26e348 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1627,6 +1627,7 @@ static u64 __kvm_read_sanitised_id_reg(const struct kvm_vcpu *vcpu,
> 		break;
> 	case SYS_ID_AA64MMFR2_EL1:
> 		val &= ~ID_AA64MMFR2_EL1_CCIDX_MASK;
> +		val &= ~ID_AA64MMFR2_EL1_NV;
> 		break;

This would cause issues when you update the host kernel while keeping the
guests register state. Could we allow to write (but ignore) the previously
valid value? Like it was handled in:
 	6685f5d572c2 KVM: arm64: Disable MPAM visibility by default and ignore VMM writes

Sebastian