queue: Add support for big-endian arches

According to the virtio specification, on non-legacy devices the
values in the virtqueue are represented in little-endian, no matter
the host nor the guest actual endianess.

If needed, fix the endianess of values that have been read from memory
or will be written to it. The methods "from_le" and "to_le" are a
no-op on little-endian machines, so this shouldn't have a performance
impact on those.

Fixes #117

Signed-off-by: Sergio Lopez <[email protected]>

crates/virtio-queue/src/lib.rs

@@ -72,6 +72,8 @@ impl Display for Error {
 impl std::error::Error for Error {}
 
 /// A virtio descriptor constraints with C representation.
+///
+/// All its fields are represented with little-endian ordering.
 #[repr(C)]
 #[derive(Default, Clone, Copy, Debug)]
 pub struct Descriptor {
@@ -95,32 +97,32 @@ impl Descriptor {
     #[cfg(any(test, feature = "test-utils"))]
     pub fn new(addr: u64, len: u32, flags: u16, next: u16) -> Self {
         Descriptor {
-            addr,
-            len,
-            flags,
-            next,
+            addr: u64::to_le(addr),
+            len: u32::to_le(len),
+            flags: u16::to_le(flags),
+            next: u16::to_le(next),
         }
     }
 
     /// Return the guest physical address of descriptor buffer
     pub fn addr(&self) -> GuestAddress {
-        GuestAddress(self.addr)
+        GuestAddress(u64::from_le(self.addr))
     }
 
     /// Return the length of descriptor buffer
     pub fn len(&self) -> u32 {
-        self.len
+        u32::from_le(self.len)
     }
 
     /// Return the flags for this descriptor, including next, write and indirect
     /// bits
     pub fn flags(&self) -> u16 {
-        self.flags
+        u16::from_le(self.flags)
     }
 
     /// Return the value stored in the `next` field of the descriptor.
     pub fn next(&self) -> u16 {
-        self.next
+        u16::from_le(self.next)
     }
 
     /// Check whether this descriptor refers to a buffer containing an indirect descriptor table.
@@ -370,6 +372,7 @@ impl<'b, M: GuestAddressSpace> Iterator for AvailIter<'b, M> {
         let head_index: u16 = self
             .mem
             .load(addr, Ordering::Acquire)
+            .map(u16::from_le)
             .map_err(|_| error!("Failed to read from memory {:x}", addr.raw_value()))
             .ok()?;
 
@@ -385,6 +388,8 @@ impl<'b, M: GuestAddressSpace> Iterator for AvailIter<'b, M> {
 }
 
 /// Represents the contents of an element from the used virtqueue ring.
+///
+/// All its fields are represented with little-endian ordering.
 #[repr(C)]
 #[derive(Clone, Copy, Default, Debug)]
 pub struct VirtqUsedElem {
@@ -394,10 +399,10 @@ pub struct VirtqUsedElem {
 
 impl VirtqUsedElem {
     /// Create a new `VirtqUsedElem` instance.
-    pub fn new(id: u16, len: u32) -> Self {
+    pub fn new(id: u32, len: u32) -> Self {
         VirtqUsedElem {
-            id: u32::from(id),
-            len,
+            id: u32::to_le(id),
+            len: u32::to_le(len),
         }
     }
 }
@@ -580,12 +585,13 @@ impl<M: GuestAddressSpace> QueueState<M> {
         let offset = VIRTQ_USED_RING_HEADER_SIZE + elem_sz;
         let addr = self.used_ring.unchecked_add(offset);
 
-        mem.store(val, addr, order).map_err(Error::GuestMemory)
+        mem.store(u16::to_le(val), addr, order)
+            .map_err(Error::GuestMemory)
     }
 
     // Set the value of the `flags` field of the used ring, applying the specified ordering.
     fn set_used_flags(&mut self, mem: &M::T, val: u16, order: Ordering) -> Result<(), Error> {
-        mem.store(val, self.used_ring, order)
+        mem.store(u16::to_le(val), self.used_ring, order)
             .map_err(Error::GuestMemory)
     }
 
@@ -630,6 +636,7 @@ impl<M: GuestAddressSpace> QueueState<M> {
         let used_event_addr = self.avail_ring.unchecked_add(offset);
 
         mem.load(used_event_addr, order)
+            .map(u16::from_le)
             .map(Wrapping)
             .map_err(Error::GuestMemory)
     }
@@ -779,6 +786,7 @@ impl<M: GuestAddressSpace> QueueStateT<M> for QueueState<M> {
         let addr = self.avail_ring.unchecked_add(2);
 
         mem.load(addr, order)
+            .map(u16::from_le)
             .map(Wrapping)
             .map_err(Error::GuestMemory)
     }
@@ -796,13 +804,13 @@ impl<M: GuestAddressSpace> QueueStateT<M> for QueueState<M> {
         let elem_sz = next_used_index * VIRTQ_USED_ELEMENT_SIZE;
         let offset = VIRTQ_USED_RING_HEADER_SIZE + elem_sz;
         let addr = self.used_ring.unchecked_add(offset);
-        mem.write_obj(VirtqUsedElem::new(head_index, len), addr)
+        mem.write_obj(VirtqUsedElem::new(head_index.into(), len), addr)
             .map_err(Error::GuestMemory)?;
 
         self.next_used += Wrapping(1);
 
         mem.store(
-            self.next_used.0,
+            u16::to_le(self.next_used.0),
             self.used_ring.unchecked_add(2),
             Ordering::Release,
         )