Re: [PATCH v3 11/28] KVM: arm64: Expose FEAT_NV2p1 to NV guests

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] The ARM64_HAS_NV2P1 CPU feature capability is checked dynamically to enable FEAT_NV2p1 logic in KVM, but the capability is never registered in the kernel's CPU feature framework, rendering the new feature entirely dead code.
--

--- Patch [11]: [PATCH v3 11/28] KVM: arm64: Expose FEAT_NV2p1 to NV guests ---
commit 2bb48d2c7459805b9d877b250f95afb9c6786a43
Author: Marc Zyngier <maz@kernel.org>

KVM: arm64: Expose FEAT_NV2p1 to NV guests

This commit exposes the FEAT_NV2p1 feature to nested virtualization guests by
updating the ID_AA64MMFR4_EL1 register limits when the hardware capability is
present. This intends to reduce the number of traps for NV guests.

> diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
> index fb54f6dad995c..82be8bcb07035 100644
> --- a/arch/arm64/kvm/nested.c
> +++ b/arch/arm64/kvm/nested.c
[ ... ]
> @@ -1738,7 +1738,11 @@ u64 limit_nv_id_reg(struct kvm *kvm, u32 reg, u64 val)
>  		if (test_bit(KVM_ARM_VCPU_HAS_EL2_E2H0, kvm->arch.vcpu_features)) {
>  			val = 0;
>  		} else {
> -			val = SYS_FIELD_PREP_ENUM(ID_AA64MMFR4_EL1, NV_frac, NV2_ONLY);
> +			val &= ID_AA64MMFR4_EL1_NV_frac;
> +			if (cpus_have_final_cap(ARM64_HAS_NV2P1))

[Severity: Medium]
Is ARM64_HAS_NV2P1 registered in the kernel's CPU feature framework?

It appears that the ARM64_HAS_NV2P1 capability is missing from the
arm64_features array in arch/arm64/kernel/cpufeature.c. Because it is not
mapped to a hardware ID register field, this capability check in
limit_nv_id_reg() will always evaluate to false.

Does this render the newly added FEAT_NV2p1 support unreachable?

> +				val = ID_REG_LIMIT_FIELD_ENUM(val, ID_AA64MMFR4_EL1, NV_frac, NV2P1);
> +			else
> +				val = SYS_FIELD_PREP_ENUM(ID_AA64MMFR4_EL1, NV_frac, NV2_ONLY);
>  			val |= SYS_FIELD_PREP_ENUM(ID_AA64MMFR4_EL1, E2H0, NI_NV1);
>  		}
>  		break;

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260722074234.3330999-1-maz@kernel.org?part=11