Re: [PATCH 1/4] KVM: arm64: Allow userspace to change MIDR_EL1

[Sebastian Ott]

Hi Oliver,

On Mon, 10 Feb 2025, Oliver Upton wrote:
> On Mon, Feb 10, 2025 at 04:49:50PM +0100, Sebastian Ott wrote:
> > Enable VMMs to write MIDR_EL1 by treating it as a VM ID register.
> > Since MIDR_EL1 is not handled as a proper arm64_ftr_reg apply only
> > a sanity check against the writable mask to ensure the reserved
> > bits are 0.
>
> How exactly does the VMM's MIDR_EL1 find its way to the guest? VPIDR_EL2
> is still set to the hardware value.

Ouch. Completely missed that part, sry.

> > @@ -2021,7 +2037,11 @@ static int set_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
> >  		return ret;
> >  	}
> >
> > -	ret = arm64_check_features(vcpu, rd, val);
> > +	if (skip_feature_check(id))
> > +		ret = arm64_check_mask(rd, val);
> > +	else
> > +		ret = arm64_check_features(vcpu, rd, val);
> > +
>
> Can you add a new implementation of ->set_user() for MIDR/REVIDR/AIDR
> instead?

Yes, sure.

> > @@ -2542,6 +2571,8 @@ static const struct sys_reg_desc sys_reg_descs[] = {
> >
> >  	{ SYS_DESC(SYS_DBGVCR32_EL2), undef_access, reset_val, DBGVCR32_EL2, 0 },
> >
> > +	{ ID_DESC(MIDR_EL1), .set_user = set_id_reg, .visibility = id_visibility,
> > +	  .reset = reset_midr_el1, .val = (u32)-1 },
>
> nit: GENMASK() instead of truncation by casting.

All done. I add a test and send out V2.
Thanks a lot!
Sebastian