[PATCH v2 4/5] KVM: arm64: Report optional ID register traps with a 0x18 syndrome

[Yao Yuan]

On Wed, Nov 26, 2025 at 03:59:50PM +0800, Marc Zyngier wrote:
> With FEAT_IDST, unimplemented system registers in the feature ID space
> must be reported using EC=0x18 at the closest handling EL, rather than
> with an UNDEF.
>
> Most of these system registers are always implemented thanks to their
> dependency on FEAT_AA64, except for a set of (currently) three registers:
> GMID_EL1 (depending on MTE2), CCSIDR2_EL1 (depending on FEAT_CCIDX),
> and SMIDR_EL1 (depending on SME).
>
> For these three registers, report their trap as EC=0x18 if they
> end-up trapping into KVM and that FEAT_IDST is not implemented in the
> guest. Otherwise, just make them UNDEF.
>
> Signed-off-by: Marc Zyngier <maz at kernel.org>
> ---
>  arch/arm64/kvm/sys_regs.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 2ca6862e935b5..7705f703e7c6d 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -82,6 +82,16 @@ static bool write_to_read_only(struct kvm_vcpu *vcpu,
>  			"sys_reg write to read-only register");
>  }
>
> +static bool idst_access(struct kvm_vcpu *vcpu, struct sys_reg_params *p,
> +			const struct sys_reg_desc *r)
> +{
> +	if (kvm_has_feat_enum(vcpu->kvm, ID_AA64MMFR2_EL1, IDS, 0x0))

Hi Marc,

Minor: maybe beter readability if use NI instead of 0x0, just like
things in feat_nv2() below, but depends on you.

static bool feat_nv2(struct kvm *kvm)
{
	return ((kvm_has_feat(kvm, ID_AA64MMFR4_EL1, NV_frac, NV2_ONLY) &&
		 kvm_has_feat_enum(kvm, ID_AA64MMFR2_EL1, NV, NI)) ||
		kvm_has_feat(kvm, ID_AA64MMFR2_EL1, NV, NV2));
}


For others(except the "not" mentioned by Ben):

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

> +		return undef_access(vcpu, p, r);
> +
> +	kvm_inject_sync(vcpu, kvm_vcpu_get_esr(vcpu));
> +	return false;
> +}
> +
>  enum sr_loc_attr {
>  	SR_LOC_MEMORY	= 0,	  /* Register definitely in memory */
>  	SR_LOC_LOADED	= BIT(0), /* Register on CPU, unless it cannot */
> @@ -3399,9 +3409,9 @@ static const struct sys_reg_desc sys_reg_descs[] = {
>  	{ SYS_DESC(SYS_CCSIDR_EL1), access_ccsidr },
>  	{ SYS_DESC(SYS_CLIDR_EL1), access_clidr, reset_clidr, CLIDR_EL1,
>  	  .set_user = set_clidr, .val = ~CLIDR_EL1_RES0 },
> -	{ SYS_DESC(SYS_CCSIDR2_EL1), undef_access },
> -	{ SYS_DESC(SYS_GMID_EL1), undef_access },
> -	{ SYS_DESC(SYS_SMIDR_EL1), undef_access },
> +	{ SYS_DESC(SYS_CCSIDR2_EL1), idst_access },
> +	{ SYS_DESC(SYS_GMID_EL1), idst_access },
> +	{ SYS_DESC(SYS_SMIDR_EL1), idst_access },
>  	IMPLEMENTATION_ID(AIDR_EL1, GENMASK_ULL(63, 0)),
>  	{ SYS_DESC(SYS_CSSELR_EL1), access_csselr, reset_unknown, CSSELR_EL1 },
>  	ID_FILTERED(CTR_EL0, ctr_el0,
> --
> 2.47.3
>