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devices: virtio: improve the comment on a memory fence in Queue::peek

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Improve the comment on why a fence is necessary in the middle of
`Queue::peek` so that it gets clearer.

BUG=b:258630435
TEST=`./tools/dev_container ./tools/presubmit` passed

Change-Id: I63f80975e6d04665948a6036998e7ed6e24be64c
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/4045661
Reviewed-by: Keiichi Watanabe <keiichiw@chromium.org>
Commit-Queue: Keiichi Watanabe <keiichiw@chromium.org>
This commit is contained in:
Takaya Saeki 2022-11-22 11:59:38 +09:00 committed by crosvm LUCI
parent 59530f1e1a
commit 984a2f7c8b
1 changed files with 3 additions and 1 deletions
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4
devices/src/virtio/queue.rs
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@ -628,7 +628,9 @@ impl Queue {
return None; return None;
} }
// so that following read from the descriptor doesn't pass get_avail_index // This fence ensures that subsequent reads from the descriptor do not
// get reordered and happen only after fetching the available_index and
// checking that there is a slot available.
fence(Ordering::SeqCst); fence(Ordering::SeqCst);
let desc_idx_addr_offset = 4 + (u64::from(self.next_avail.0 % queue_size) * 2); let desc_idx_addr_offset = 4 + (u64::from(self.next_avail.0 % queue_size) * 2);