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Differential D55943

arm64: Handle changing self-referential DMAP pages
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Authored by andrew on Mar 19 2026, 4:04 PM.
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manu
alc
kib
markj
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arm64
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rG580958427536: arm64: Handle changing self-referential DMAP pages
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

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rG FreeBSD src repository
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Event Timeline

andrew created this revision.Mar 19 2026, 4:04 PM
Herald added a reviewer: manu. · View Herald TranscriptMar 19 2026, 4:04 PM
Herald added subscribers: emaste, imp. · View Herald Transcript
andrew requested review of this revision.Mar 19 2026, 4:04 PM
Harbormaster completed remote builds in B71524: Diff 173913.Mar 19 2026, 4:04 PM
andrew added a child revision: D55944: arm64: mte: cleanup cache register definitions.Mar 19 2026, 4:04 PM
andrew added reviewers: alc, kib, markj.Mar 19 2026, 4:25 PM
kib added inline comments.Mar 19 2026, 10:54 PM
sys/arm64/arm64/pmap.c
8384

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 added inline comments.Mar 20 2026, 5:11 AM
sys/arm64/arm64/pmap.c
8384

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

andrew added inline comments.Mar 23 2026, 9:51 AM
sys/arm64/arm64/pmap.c
8384

We already return a failure if demote fails above.

markj added inline comments.Mar 23 2026, 2:52 PM
sys/arm64/arm64/pmap.c
8384

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.

andrew updated this revision to Diff 175105.Wed, Apr 8, 1:03 PM

Use the CMAP from D56306

Harbormaster completed remote builds in B72053: Diff 175105.Wed, Apr 8, 1:03 PM
andrew added a parent revision: D56306: arm64: Add a cmap page to pmap.Wed, Apr 8, 1:04 PM
andrew updated this revision to Diff 175227.Fri, Apr 10, 2:23 PM

Update after D56306 changes

Harbormaster completed remote builds in B72106: Diff 175227.Fri, Apr 10, 2:23 PM
kib accepted this revision.Fri, Apr 10, 2:33 PM
This revision is now accepted and ready to land.Fri, Apr 10, 2:33 PM
Closed by commit rG580958427536: arm64: Handle changing self-referential DMAP pages (authored by andrew). · Explain WhyMon, Apr 13, 2:24 PM
This revision was automatically updated to reflect the committed changes.
andrew added a commit: rG580958427536: arm64: Handle changing self-referential DMAP pages.

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sys/
arm64/
arm64/
26 lines

Diff 175431

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sys/arm64/arm64/pmap.c

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