Re: [PATCH] KVM: arm64: nv: Exclude guest's TWED configuration when TWE isn't set

[Marc Zyngier <maz@kernel.org>]

On Tue, 09 Sep 2025 08:16:02 +0100,
Oliver Upton <oliver.upton@linux.dev> wrote:
> 
> KVM blindly OR's the guest values for TWEDEL and TWEDEn into the
> programmed value even if the guest has WFE traps disabled. Exclude these
> fields from the final value when TWE isn't set to avoid potentially
> delaying a trap desired by the host.
> 
> Note that KVM doesn't use FEAT_TWED currently so there's no need to
> empty out these fields the other way around (i.e. host TWE=0, guest
> TWE=1).
> 
> Fixes: 04ab519bb86d ("KVM: arm64: nv: Configure HCR_EL2 for FEAT_NV2")
> Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
> ---
>  arch/arm64/kvm/hyp/vhe/switch.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/arch/arm64/kvm/hyp/vhe/switch.c b/arch/arm64/kvm/hyp/vhe/switch.c
> index 0998ad4a2552..e4955b49c0d2 100644
> --- a/arch/arm64/kvm/hyp/vhe/switch.c
> +++ b/arch/arm64/kvm/hyp/vhe/switch.c
> @@ -95,6 +95,13 @@ static u64 __compute_hcr(struct kvm_vcpu *vcpu)
>  			/* Force NV2 in case the guest is forgetful... */
>  			guest_hcr |= HCR_NV2;
>  		}
> +
> +		/*
> +		 * Exclude the guest's TWED configuration if it hasn't set TWE
> +		 * to avoid potentially delaying traps for the host.
> +		 */
> +		if (!(guest_hcr & HCR_TWE))
> +			guest_hcr &= (HCR_EL2_TWEDEn | HCR_EL2_TWEDEL);
>  	}
>  
>  	BUG_ON(host_data_test_flag(VCPU_IN_HYP_CONTEXT) &&
> 
> base-commit: b320789d6883cc00ac78ce83bccbfe7ed58afcf0

I'm a bit lost here.

We seem to sanitise ID_AA64MMFR1_EL1 in a way that totally removes
TWED from the feature set (see limit_nv_id_reg()). Therefore,
HCR_EL2.(TWEDEn,TWEDEL} should be RES0, no matter what.

Is there another way the guest can set these bits?

	M.

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