google / alioth / 37276303154

39%

main: 38%

Build Type

push

github

Committed by Lencerf

Commit Message

refactor(mem): turn RamBus into an RCU cell of Ram snapshots

Virtio workers used to hold RamBus::lock_layout() for as long as a
device was running, so any runtime change of the guest RAM layout,
e.g. memory hot-plug, blocked forever on the write lock.

Borrowing the read-copy-update idea from Linux RCU,

- Readers call RamBus::load() to get an immutable snapshot. The read
  lock is only held while cloning the Arc.
- Writers call RamBus::update(), which copies the layout, applies the
  change and publishes the result atomically. A failed update
  publishes nothing, and Memory batches all RAM ranges of a region
  into a single update.
- Each published layout carries a generation number, so a reader can
  detect a stale snapshot with a single atomic load by comparing
  Ram::generation() with RamBus::generation().

Since ArcMemPages is reference counted, a snapshot keeps every host
mapping it refers to alive, even after the slot is removed from the
bus.

This commit fixes the deadlock, but a running virtio worker still uses
the snapshot taken when the device was activated. Follow-up commits
make workers pick up new snapshots.

Assisted-by: Antigravity:Claude-Fable-5.1
Signed-off-by: Changyuan Lyu <changyuanl@google.com>

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2 existing lines in 1 file now uncovered.

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