Re: [PATCH v5 4/5] KVM: selftests: arm64: Skip all 32 bit IDs when set_id_regs is aarch64 only

On Tue, Mar 24, 2026 at 04:47:15PM +0000, Marc Zyngier wrote:

Mark Brown [off-list ref] wrote:

quoted

On an aarch64 only system the 32 bit ID registers have UNDEFINED values.
As a result set_id_regs skips tests for setting fields in these registers
when testing an aarch64 only guest. This has the side effect of meaning
that we don't record an expected value for these registers, meaning that
when the subsequent tests for values being visible in guests and preserved
over reset check the value they can spuriously fail. This can be seen by
running on an emulated system with both NV and 32 bit enabled, NV will
result in the guests created by the test program being 64 bit only but
the 32 bit ID registers will have values.

I don't think papering over this problem is the right thing to do.

If the issue is that you have HW that has both NV and AArch32, then
KVM needs to be fixed to make the 32bit IDregs RAZ when NV is present
because that's not a configuration we support.

Yes, I'm seeing this in practice - it hits by default with qemu which is
rather more readily accessible than actual hardware with NV at this
point and I was concerned that since these registers are explicitly
UNKNOWN (sorry, a mistake in the commit log above there) rather than RAZ
if FEAT_AA32 is not implemented there might be gotchas.  I do see that
there's some forcing for the case where the host doesn't support
FEAT_AA32, I figured there must be some reason why it wasn't done based
on the guest configuration.

I'll add something to kvm_finalize_sys_regs(), it'll still need updates
in set_id_regs since that'd make the 32 bit ID registers change value
when the guest is run.