block, virtio-devices: Support request submission in batch

Cache and batch IO requests after parsing all
items in the queue, improving performance—especially
for small block sizes—by reducing per-request overhead.

Introduced two methods in the AsyncIo trait for batch
submission, with implementation in the raw disk backend.
This method should be called during/after parsing all block IO requests
in the available queue. If the batch submission is not enabled, by
default it does the old way of submitting requests.

Signed-off-by: Bo Chen <bchen@crusoe.ai>
Signed-off-by: Muminul Islam <muislam@microsoft.com>

block/src/async_io.rs

@@ -8,7 +8,7 @@ use std::os::fd::{AsRawFd, OwnedFd, RawFd};
 use thiserror::Error;
 use vmm_sys_util::eventfd::EventFd;
 
-use crate::DiskTopology;
+use crate::{BatchRequest, DiskTopology};
 
 #[derive(Error, Debug)]
 pub enum DiskFileError {
@@ -99,4 +99,10 @@ pub trait AsyncIo: Send {
     ) -> AsyncIoResult<()>;
     fn fsync(&mut self, user_data: Option<u64>) -> AsyncIoResult<()>;
     fn next_completed_request(&mut self) -> Option<(u64, i32)>;
+    fn batch_requests_enabled(&self) -> bool {
+        false
+    }
+    fn submit_batch_requests(&mut self, _batch_request: &[BatchRequest]) -> AsyncIoResult<()> {
+        Ok(())
+    }
 }