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

From: Oliver Upton <[email-masked]>
Date: Wed, 26 Nov 2025 22:43:51 -0800

On Thu, Nov 27, 2025 at 02:07:08PM +0800, Yao Yuan wrote:
> 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 <[email-masked]>
> > ---
> >  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.

+1, using the ESR value as an identifier in the sysreg tables is just
terrible. This reads like a literal.

Thanks,
Oliver