add QueueStateT::pop_descriptor_chain

This commit also introduces a relaxation of the trait bound on the
generic type parameter `M` for `QueueStateT::avail_idx` (i.e. it no
longer has to be `Sized`, as it was implicitly the case before). This
change reduces the number of trait bounds that have to be explicitly
propagated to the newly introduced methods. We should apply a similar
update to the rest of the interface as well (tracked by rust-vmm#152).

Signed-off-by: Alexandru Agache <[email protected]>

crates/virtio-queue/src/iterator.rs

@@ -102,7 +102,7 @@ pub struct AvailIter<'b, M> {
 impl<'b, M> AvailIter<'b, M>
 where
     M: Deref,
-    M::Target: GuestMemory + Sized,
+    M::Target: GuestMemory,
 {
     /// Create a new instance of `AvailInter`.
     ///

crates/virtio-queue/src/lib.rs

@@ -155,7 +155,9 @@ pub trait QueueStateT: for<'a> QueueStateGuard<'a> {
     fn set_event_idx(&mut self, enabled: bool);
 
     /// Read the `idx` field from the available ring.
-    fn avail_idx<M: GuestMemory>(&self, mem: &M, order: Ordering) -> Result<Wrapping<u16>, Error>;
+    fn avail_idx<M>(&self, mem: &M, order: Ordering) -> Result<Wrapping<u16>, Error>
+    where
+        M: GuestMemory + ?Sized;
 
     /// Read the `idx` field from the used ring.
     fn used_idx<M: GuestMemory>(&self, mem: &M, order: Ordering) -> Result<Wrapping<u16>, Error>;
@@ -193,4 +195,14 @@ pub trait QueueStateT: for<'a> QueueStateGuard<'a> {
 
     /// Set the index for the next descriptor in the used ring.
     fn set_next_used(&mut self, next_used: u16);
+
+    /// Pop and return the next available descriptor chain, or `None` when there are no more
+    /// descriptor chains available.
+    ///
+    /// This enables the consumption of available descriptor chains in a "one at a time"
+    /// manner, without having to hold a borrow after the method returns.
+    fn pop_descriptor_chain<M>(&mut self, mem: M) -> Option<DescriptorChain<M>>
+    where
+        M: Clone + Deref,
+        M::Target: GuestMemory;
 }

crates/virtio-queue/src/state.rs

@@ -19,7 +19,10 @@ use crate::defs::{
     VIRTQ_USED_ELEMENT_SIZE, VIRTQ_USED_F_NO_NOTIFY, VIRTQ_USED_RING_HEADER_SIZE,
     VIRTQ_USED_RING_META_SIZE,
 };
-use crate::{error, AvailIter, Descriptor, Error, QueueStateGuard, QueueStateT, VirtqUsedElem};
+use crate::{
+    error, AvailIter, Descriptor, DescriptorChain, Error, QueueStateGuard, QueueStateT,
+    VirtqUsedElem,
+};
 
 /// Struct to maintain information and manipulate state of a virtio queue.
 ///
@@ -71,7 +74,7 @@ impl QueueState {
     pub fn iter<M>(&mut self, mem: M) -> Result<AvailIter<'_, M>, Error>
     where
         M: Deref,
-        M::Target: GuestMemory + Sized,
+        M::Target: GuestMemory,
     {
         self.avail_idx(mem.deref(), Ordering::Acquire)
             .map(move |idx| AvailIter::new(mem, idx, self))
@@ -311,7 +314,10 @@ impl QueueStateT for QueueState {
         self.event_idx_enabled = enabled;
     }
 
-    fn avail_idx<M: GuestMemory>(&self, mem: &M, order: Ordering) -> Result<Wrapping<u16>, Error> {
+    fn avail_idx<M>(&self, mem: &M, order: Ordering) -> Result<Wrapping<u16>, Error>
+    where
+        M: GuestMemory + ?Sized,
+    {
         let addr = self
             .avail_ring
             .checked_add(2)
@@ -450,4 +456,19 @@ impl QueueStateT for QueueState {
     fn set_next_used(&mut self, next_used: u16) {
         self.next_used = Wrapping(next_used);
     }
+
+    fn pop_descriptor_chain<M>(&mut self, mem: M) -> Option<DescriptorChain<M>>
+    where
+        M: Clone + Deref,
+        M::Target: GuestMemory,
+    {
+        // Default, iter-based impl. Will be subsequently improved.
+        match self.iter(mem) {
+            Ok(mut iter) => iter.next(),
+            Err(e) => {
+                error!("Iterator error {}", e);
+                None
+            }
+        }
+    }
 }

crates/virtio-queue/src/state_sync.rs

@@ -3,12 +3,13 @@
 // SPDX-License-Identifier: Apache-2.0 AND BSD-3-Clause
 
 use std::num::Wrapping;
+use std::ops::Deref;
 use std::sync::atomic::Ordering;
 use std::sync::{Arc, Mutex, MutexGuard};
 
 use vm_memory::GuestMemory;
 
-use crate::{Error, QueueState, QueueStateGuard, QueueStateT};
+use crate::{DescriptorChain, Error, QueueState, QueueStateGuard, QueueStateT};
 
 /// Struct to maintain information and manipulate state of a virtio queue for multi-threaded
 /// context.
@@ -107,7 +108,10 @@ impl QueueStateT for QueueStateSync {
         self.lock_state().set_event_idx(enabled);
     }
 
-    fn avail_idx<M: GuestMemory>(&self, mem: &M, order: Ordering) -> Result<Wrapping<u16>, Error> {
+    fn avail_idx<M>(&self, mem: &M, order: Ordering) -> Result<Wrapping<u16>, Error>
+    where
+        M: GuestMemory + ?Sized,
+    {
         self.lock_state().avail_idx(mem, order)
     }
 
@@ -151,6 +155,14 @@ impl QueueStateT for QueueStateSync {
     fn set_next_used(&mut self, next_used: u16) {
         self.lock_state().set_next_used(next_used);
     }
+
+    fn pop_descriptor_chain<M>(&mut self, mem: M) -> Option<DescriptorChain<M>>
+    where
+        M: Clone + Deref,
+        M::Target: GuestMemory,
+    {
+        self.lock_state().pop_descriptor_chain(mem)
+    }
 }
 
 #[cfg(test)]