[RFC PATCH v2 0/3] Set DBM bit on writable pages

[Leonardo Bras]

When HAFDBS is enabled (VTCR_EL2.{HA,HD}={1,1}), having a DBM bit allows 
hardware to promote a non-writable page to writable by setting (S2AP[1]=1),
without having to take a fault.

The reason for this patchset is to add the DBM bit when setting-up writable 
pages, so the code is prepared for future patches that make use of 
HDBSS/HACDBS to accelerate dirty-tracking. Write-bit detection still 
happens based only in S2AP[1], so no change in that aspect.

Having DBM set on block entries is undesired, as we benefit from the first 
fault for splitting it in PAGE_SIZE entries, which is the granularity used 
to track the dirty pages in KVM. So introduce a new walker that, on top of 
clearing S2AP[1] for every entry, also clear DBM for block entries.

Also introduce a new VCPU request to reload Stage2, as it will be needed on 
future HDBSS/HAFDBS patchsets. 

Please let me know of what do you think!

Changes since RFCv1:
- Dropped patch #5 on enabling HAFDBS on non-dirty-tracking guests, as it 
  proved to be counter-productive on wrprotect, contrary to expectations
- No more moving writable-bit to DBM. That does not seem to be the intended 
  use. It makes sense and it's easier to convert code to new DBM approach.
- No new PROT_DIRTY, so drop patch #2
- Tried adding a PROT_DBM, but does not seem necessary to fault a page as 
  writable-clean. 
- KVM_REQ_RELOAD_STAGE2 now depends on VHE, as it already reloads Stage2 on 
  guest entry otherwise. 
Link: https://lore.kernel.org/all/20260901171558.2674031-1-leo.bras@xxxxxxx/

Thanks!
Leo

Leonardo Bras (3):
  KVM: arm64: pgtables: Set DBM attribute on writable Stage-2 PTEs
  KVM: arm64: Introduce a dedicated walker for stage2 write-protect
  KVM: arm64: Add KVM_REQ_RELOAD_STAGE2

 arch/arm64/include/asm/kvm_host.h    |  2 ++
 arch/arm64/include/asm/kvm_pgtable.h |  3 +++
 arch/arm64/kvm/arm.c                 | 10 ++++++++++
 arch/arm64/kvm/hyp/pgtable.c         | 30 ++++++++++++++++++++++------
 4 files changed, 39 insertions(+), 6 deletions(-)


base-commit: c03c1a14507dfaf8b856c2608f9ac216021fc222
-- 
2.56.0