Subject: Re: [PATCH v2 2/5] KVM: arm64: Force trap of GMID_EL1 when the guest doesn't have MTE

From: Oliver Upton <[email-masked]>
Date: Wed, 26 Nov 2025 22:37:13 -0800

On Wed, Nov 26, 2025 at 03:59:48PM +0000, Marc Zyngier wrote:
> If our host has MTE, but the guest doesn't, make sure we set HCR_EL2.TID5
> to force GMID_EL1 being trapped.
> 
> Reviewed-by: Joey Gouly <[email-masked]>
> Signed-off-by: Marc Zyngier <[email-masked]>
> ---
>  arch/arm64/kvm/sys_regs.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 9e4c46fbfd802..2ca6862e935b5 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -5561,6 +5561,8 @@ static void vcpu_set_hcr(struct kvm_vcpu *vcpu)
>  
>  	if (kvm_has_mte(vcpu->kvm))
>  		vcpu->arch.hcr_el2 |= HCR_ATA;
> +	else if (id_aa64pfr1_mte(read_sanitised_ftr_reg(SYS_ID_AA64PFR1_EL1)))

This helper is ugly!

> +		vcpu->arch.hcr_el2 |= HCR_TID5;

How about setting the trap unconditionally when !kvm_has_mte()? Even in
the case of asymmetry we'd want GMID_EL1 to trap.

Thanks,
Oliver