virtio_queue: Add descriptors_utils.rs

This commit adds descriptors_utils.rs from virtiofsd. This module
provides the Reader/Writer classes to iterate over descriptors. Devices
shall rely on this interface instead of manipulate descriptors. The
version in this commit removes read_to_at() and write_from_at() that are
required only for virtiofsd. To instantiate these helpers, user shall
use the reader()/writer() functions defined in the context of
DescriptorChain.

Co-developed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Matias Ezequiel Vara Larsen <mvaralar@redhat.com>

virtio-queue/src/chain.rs

@@ -14,9 +14,10 @@ use std::fmt::{self, Debug};
 use std::mem::size_of;
 use std::ops::Deref;
 
-use vm_memory::{Address, Bytes, GuestAddress, GuestMemory};
+use vm_memory::bitmap::{BitmapSlice, WithBitmapSlice};
+use vm_memory::{Address, Bytes, GuestAddress, GuestMemory, GuestMemoryRegion};
 
-use crate::{Descriptor, Error};
+use crate::{Descriptor, Error, Reader, Writer};
 use virtio_bindings::bindings::virtio_ring::VRING_DESC_ALIGN_SIZE;
 
 /// A virtio descriptor chain.
@@ -88,6 +89,24 @@ where
         }
     }
 
+    /// Return a new instance of Writer
+    pub fn writer<'a, B: BitmapSlice>(self, mem: &'a M::Target) -> Result<Writer<'a, B>, Error>
+    where
+        M::Target: Sized,
+        <<M::Target as GuestMemory>::R as GuestMemoryRegion>::B: WithBitmapSlice<'a, S = B>,
+    {
+        Writer::new(mem, self).map_err(|_| Error::InvalidChain)
+    }
+
+    /// Return a new instance of Reader
+    pub fn reader<'a, B: BitmapSlice>(self, mem: &'a M::Target) -> Result<Reader<'a, B>, Error>
+    where
+        M::Target: Sized,
+        <<M::Target as GuestMemory>::R as GuestMemoryRegion>::B: WithBitmapSlice<'a, S = B>,
+    {
+        Reader::new(mem, self).map_err(|_| Error::InvalidChain)
+    }
+
     /// Return an iterator that only yields the writable descriptors in the chain.
     pub fn writable(self) -> DescriptorChainRwIter<M> {
         DescriptorChainRwIter {