descriptor_utils: Remove need for temporary vectors

Refactor the DescriptorChainConsumer, Reader, and Writer structs so that
we don't have to allocate a Vec on the heap every time we read or write
from a DescriptorChain.  This should hopefully give us some small
performance improvements as well as simplify the code in some places.

Also switch from VolatileSlices to iovecs so that it's easier to use
these structs with io_uring.  Otherwise we would end up allocating
temporary vectors to convert from VolatlieSlice to iovec.

BUG=none
TEST=unit tests

Change-Id: I1657bc76cfff084df825dbbdc8ff414740b71a8f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2190106
Reviewed-by: Dylan Reid <dgreid@chromium.org>
Reviewed-by: Daniel Verkamp <dverkamp@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>
Commit-Queue: Dylan Reid <dgreid@chromium.org>

1 files changed, 2 insertions, 7 deletions

diff --git a/devices/src/virtio/fs/server.rs b/devices/src/virtio/fs/server.rs
index 33b7c98..c1af80c 100644
--- a/devices/src/virtio/fs/server.rs
+++ b/devices/src/virtio/fs/server.rs
@@ -496,10 +496,7 @@ impl<F: FileSystem + Sync> Server<F> {
         };
 
         // Split the writer into 2 pieces: one for the `OutHeader` and the rest for the data.
-        let data_writer = ZCWriter(
-            w.split_at(size_of::<OutHeader>())
-                .map_err(Error::InvalidDescriptorChain)?,
-        );
+        let data_writer = ZCWriter(w.split_at(size_of::<OutHeader>()));
 
         match self.fs.read(
             Context::from(in_header),
@@ -910,9 +907,7 @@ impl<F: FileSystem + Sync> Server<F> {
         }
 
         // Skip over enough bytes for the header.
-        let mut cursor = w
-            .split_at(size_of::<OutHeader>())
-            .map_err(Error::InvalidDescriptorChain)?;
+        let mut cursor = w.split_at(size_of::<OutHeader>());
 
         let res = if plus {
             self.fs.readdirplus(