google / alioth / 36648321950

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push

github

Committed by Lencerf

Commit Message

fix(virtio): fix memory ordering in split and packed virtqueues

Shared virtqueue rings in guest memory are accessed concurrently by host
worker threads and guest vCPUs. Fix the memory ordering and ring access
primitives across Queue, SplitQueue, and PackedQueue:

- In Queue::handle_desc() and Queue::handle_deferred(), place a SeqCst
  fence between push_used() and interrupt_enabled() so that publishing a
  used entry is always ordered before reading the driver's interrupt
  suppression state. Without this store-load barrier, checking
  used_event or driver DescEvent can be reordered before the used
  index/flag update, losing interrupts on asynchronous completions such
  as io_uring. In handle_desc(), the fence and the check are skipped once
  an interrupt is already pending for the batch.

- Access synchronization control words with AtomicU16::from_ptr using
  Acquire loads (avail_hdr.idx, packed desc.flag, interrupt.flag) and
  Release stores (used_hdr.idx, packed used desc.flag), and use Relaxed
  atomic loads/stores for event and flag fields paired with explicit
  fences (avail_event, used_event, ring flags, notification.flag,
  interrupt.index).

- Read and write non-atomic ring payload data (descriptors, avail_ring
  entries, and used_ring elements) with read_volatile()/write_volatile()
  instead of shared/mutable Rust references, and mark PackedQueue's
  DescEvent as #[repr(C)].

Closes: #504
Assisted-by: Antigravity:Gemini-3.8-Flash
Signed-off-by: Changyuan Lyu <changyuanl@google.com>

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49 of 50 new or added lines in 3 files covered. (98.0%)

2 existing lines in 1 file now uncovered.

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