Re: [PATCH 24/28] KVM: arm64: Engage NV3 ERET trap elision

[Joey Gouly <joey.gouly@arm.com>]

On Thu, Jul 02, 2026 at 05:02:44PM +0100, Marc Zyngier wrote:
> When running on NV3 HW, always engage ERET trap elision when running
> the L1 context, as there is no benefit in not doing so.
> 
> An L1 can itself engage trap elision by setting its own view of
> HCRX_EL2.NVTGE==1, which will subsequently be honored.
> 
> Signed-off-by: Marc Zyngier <maz@kernel.org>

Reviewed-by: Joey Gouly <joey.gouly@arm.com>

> ---
>  arch/arm64/include/asm/kvm_emulate.h | 10 ++++++++++
>  arch/arm64/kvm/hyp/vhe/switch.c      |  4 ++++
>  2 files changed, 14 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
> index c562d8171d5e1..b32870a5e1236 100644
> --- a/arch/arm64/include/asm/kvm_emulate.h
> +++ b/arch/arm64/include/asm/kvm_emulate.h
> @@ -706,6 +706,16 @@ static inline void vcpu_set_hcrx(struct kvm_vcpu *vcpu)
>  
>  		if (kvm_has_feat(kvm, ID_AA64ISAR1_EL1, LS64, LS64_V))
>  			vcpu->arch.hcrx_el2 |= HCRX_EL2_EnASR;
> +
> +		/*
> +		 * NV3 is a host-specific extension, and we always use it
> +		 * when present and that the guest uses NV. It may be be
> +		 * hidden from the guest though.
> +		 */
> +		if (cpus_have_final_cap(ARM64_HAS_NV3) &&
> +		    vcpu_has_nv(vcpu) && vcpu_el2_e2h_is_set(vcpu)) {
> +			vcpu->arch.hcrx_el2 |= HCRX_EL2_NVTGE;
> +		}
>  	}
>  }
>  #endif /* __ARM64_KVM_EMULATE_H__ */
> diff --git a/arch/arm64/kvm/hyp/vhe/switch.c b/arch/arm64/kvm/hyp/vhe/switch.c
> index c5c06ae41b229..f129f22f15618 100644
> --- a/arch/arm64/kvm/hyp/vhe/switch.c
> +++ b/arch/arm64/kvm/hyp/vhe/switch.c
> @@ -345,6 +345,10 @@ static bool kvm_hyp_handle_eret(struct kvm_vcpu *vcpu, u64 *exit_code)
>  	u64 esr = kvm_vcpu_get_esr(vcpu);
>  	u64 spsr, elr, mode;
>  
> +	/* With NV3, the fast path is handled in HW */
> +	if (cpus_have_final_cap(ARM64_HAS_NV3) && vcpu_el2_e2h_is_set(vcpu))
> +		return false;
> +
>  	/*
>  	 * Going through the whole put/load motions is a waste of time
>  	 * if this is a VHE guest hypervisor returning to its own
> -- 
> 2.47.3
>