arm64: Handle changing self-referential DMAP pages

[andrew]

Summary

Support changing the property of a DMAP page that holds it's own page
table entry.

Because we need to perform a break-before-make sequence to change the
properties of pages a page that also holds it's own page table entry
will fault in the make part of the sequence.

Handle this by mapping the page with a temporary mapping as we already
do when demoting a superpage.

Sponsored by: Arm Ltd

[kib]

sys/arm64/arm64/pmap.c

8366

IMO it makes more sense to reserve a page frame and use it there instead of allocating and then freeing KVA, similar to the CMAPs on amd64. You would avoid the weird EINVAL from below.

[markj]

sys/arm64/arm64/pmap.c

8366

I agree. It would be better to avoid error cases when updating the kernel pmap.

[andrew]

sys/arm64/arm64/pmap.c

8366

We already return a failure if demote fails above.

[markj]

sys/arm64/arm64/pmap.c

8366

We can remove the first error path in pmap_demote_l2() by implementing kib's suggestion. The second one won't happen for kernel maps. The third one is quite unlikely to happen for kernel maps given that we use VM_ALLOC_INTERRUPT to allocate the new L3 page. I still think it's better to avoid the failure mode here.