refactor(queue): simplify do_pop_unchecked

Instead use `avail_ring_ring_get()`.

Signed-off-by: Egor Lazarchuk <[email protected]>

Author: ShadowCurse

src/vmm/src/devices/virtio/queue.rs

@@ -584,28 +584,13 @@ impl Queue {
         // In a naive notation, that would be:
         // `descriptor_table[avail_ring[next_avail]]`.
         //
-        // Avail ring has layout:
-        // struct AvailRing {
-        //     flags: u16,
-        //     idx: u16,
-        //     ring: [u16; <queue size>],
-        //     used_event: u16,
-        // }
-        // We calculate offset into `ring` field.
-        // We use `self.next_avail` to store the position, of the next available descriptor
-        // index in the `ring` field. Because `self.next_avail` is only incremented, the actual
-        // index into `AvailRing` is `self.next_avail % self.actual_size()`.
-        let desc_index_offset = std::mem::size_of::<u16>()
-            + std::mem::size_of::<u16>()
-            + std::mem::size_of::<u16>() * usize::from(self.next_avail.0 % self.actual_size());
-        let desc_index_address = self
-            .avail_ring_address
-            .unchecked_add(usize_to_u64(desc_index_offset));
-
-        // `self.is_valid()` already performed all the bound checks on the descriptor table
-        // and virtq rings, so it's safe to unwrap guest memory reads and to use unchecked
-        // offsets.
-        let desc_index: u16 = mem.read_obj(desc_index_address).unwrap();
+        // We use `self.next_avail` to store the position, in `ring`, of the next available
+        // descriptor index, with a twist: we always only increment `self.next_avail`, so the
+        // actual position will be `self.next_avail % self.actual_size()`.
+        let idx = self.next_avail.0 % self.actual_size();
+        // SAFETY:
+        // index is bound by the queue size
+        let desc_index = unsafe { self.avail_ring_ring_get(usize::from(idx)) };
 
         DescriptorChain::checked_new(mem, self.desc_table_address, self.actual_size(), desc_index)
             .map(|dc| {