[PATCH v3 4/9] KVM: arm64: Handle FEAT_IDST for sysregs without specific handlers

[Yao Yuan]

On Thu, Dec 04, 2025 at 09:48:01AM +0800, Marc Zyngier wrote:
> Add a bit of infrastrtcture to triage_sysreg_trap() to handle the
> case of registers falling into the Feature ID space that do not
> have a local handler.
>
> For these, we can directly apply the FEAT_IDST semantics and inject
> an EC=0x18 exception. Otherwise, an UNDEF will do.
>
> Signed-off-by: Marc Zyngier <maz at kernel.org>
> ---
>  arch/arm64/kvm/emulate-nested.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
>
> diff --git a/arch/arm64/kvm/emulate-nested.c b/arch/arm64/kvm/emulate-nested.c
> index 616eb6ad68701..fac2707221b47 100644
> --- a/arch/arm64/kvm/emulate-nested.c
> +++ b/arch/arm64/kvm/emulate-nested.c
> @@ -2588,6 +2588,26 @@ bool triage_sysreg_trap(struct kvm_vcpu *vcpu, int *sr_index)
>
>  		params = esr_sys64_to_params(esr);
>
> +		/*
> +		 * This implements the pseudocode UnimplementedIDRegister()
> +		 * helper for the purpose of fealing with FEAT_IDST.
> +		 *
> +		 * The Feature ID space is defined as the System register
> +		 * space in AArch64 with op0==3, op1=={0, 1, 3}, CRn==0,
> +		 * CRm=={0-7}, op2=={0-7}.
> +		 */
> +		if (params.Op0 == 3 &&
> +		    !(params.Op1 & 0b100) && params.Op1 != 2 &&
> +		    params.CRn == 0 &&
> +		    !(params.CRm & 0b1000)) {

Hi Marc,

May give a macro/inline function or local variable w/ name like "is_feat_id_space()"
can simplify the long comment here. Others LGTM:

Reviewed-by: Yuan Yao <yaoyuan at linux.alibaba.com>

> +			if (kvm_has_feat_enum(vcpu->kvm, ID_AA64MMFR2_EL1, IDS, IMP))
> +				kvm_inject_sync(vcpu, kvm_vcpu_get_esr(vcpu));
> +			else
> +				kvm_inject_undefined(vcpu);
> +
> +			return true;
> +		}
> +
>  		/*
>  		 * Check for the IMPDEF range, as per DDI0487 J.a,
>  		 * D18.3.2 Reserved encodings for IMPLEMENTATION
> --
> 2.47.3