[Backport 1.16] fix(vmm): commit virtio-mem block state after the KVM slot update (#6176) - #6184
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ilstam merged 4 commits intoSep 3, 2026
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In `update_range` function, move kvm slot update before memory discard to prevent a small time window when the guest can re-fault just discarded memory before it is removed from kvm view. (cherry picked from commit 8c4bc5a) [backport: prerequisite for the firecracker-microvm#6176 backport that follows] Signed-off-by: Egor Lazarchuk <yegorlz@amazon.co.uk>
update_range committed the block state (plugged_blocks, plugged_size) for the whole request up front and only then updated the KVM slots, and update_slot flipped its per-slot bitmap before issuing the KVM ioctl. On a multi-slot request that failed part way through, the committed state covered the whole range while KVM reflected only the slots that had been updated, leaving the device's bookkeeping inconsistent with KVM. Update KVM one slot at a time and commit that slot's block state only after its KVM update succeeds; discard an unplugged slot only once it is committed. A partial failure now stops with the block state and the KVM slots reflecting exactly the slots that were updated, and never discards a block that has not been committed as unplugged. In update_slot, commit the slot bitmap only once both the protection change and the KVM slot update have succeeded, and roll back the first of the two when the second fails, so a failed slot update leaves the slot exactly as it was. A failed rollback would leave the host mapping and KVM out of sync with no way back, so it panics. The whole-range discard is now done per slot as each slot commits. This issues one discard per intersecting slot instead of one for the range, which is not on a hot path. Add a test that plugs a range spanning two KVM slots with the second slot's memory-region update forced to fail, and checks that only the first slot's block is left committed. A test-only fault injector on set_user_memory_region drives the failure. (cherry picked from commit 1e99e52) [backport: depends on the KVM-first ordering from firecracker-microvm#5944, picked as the previous commit; resolved a conflict with 1.16's discard block in update_range, the result matches main] Signed-off-by: Jay Chung <jaehoc@amazon.com>
Record that virtio-mem now commits a slot's block state only after its KVM update succeeds, keeping the device's bookkeeping consistent with KVM when a plug or unplug request fails part way through. (cherry picked from commit 9384f39) Signed-off-by: Jay Chung <jaehoc@amazon.com>
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Additional details and impacted files@@ Coverage Diff @@
## firecracker-v1.16 #6184 +/- ##
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+ Coverage 83.07% 83.09% +0.02%
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Files 277 277
Lines 30228 30285 +57
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+ Hits 25111 25165 +54
- Misses 5117 5120 +3
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Manciukic
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ilstam
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Sep 3, 2026
ilstam
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September 3, 2026 13:06
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Backport #6176 into v1.16
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