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

[Ben Horgan <ben.horgan@arm.com>]

Hi Marc,

On 11/26/25 15:59, 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.

Missed it before but there is a stray 'not' in there. You get the
EC=0x18 trap if FEAT_IDST is implemented.

> 
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>  arch/arm64/kvm/sys_regs.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)

The code looks good.

Reviewed-by: Ben Horgan

Thanks,

Ben