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

[Ben Horgan]

Hi Marc,

On 11/20/25 13:32, Marc Zyngier wrote:
> With FEAT_IDST, unimplemented system registers must be reported using
> EC=0x18 at the closest handling EL, rather than with an UNDEF.

I think this needs 'in the feature ID space' adding. Something like:

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 system registers are always implemented thanks to their dependency

'Most of these system registers...'

> 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).
I agree that these 3 are currently the only optional system registers in
the feature ID space.

> 
> 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 40f32b017f107..992137822dcf9 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))
> +		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 */
> @@ -3396,9 +3406,9 @@ 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,


Thanks,

Ben