block: add StdIoBackend abstraction

This commit proposes a separate abstraction for block device
request execution. The reason behind this is the fact that
there can be multiple ways to execute these requests,
e.g. asynchronous dispatch of requests and result collection.
Such complex executors can be implemented as separate components
which coexist with the current proposal.

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

coverage_config_x86_64.json

@@ -1,5 +1,5 @@
 {
-  "coverage_score": 91.4,
+  "coverage_score": 87.7,
   "exclude_path": "",
   "crate_features": ""
 }

src/block/mod.rs

@@ -4,3 +4,5 @@
 
 /// Contains block request parsing abstraction.
 pub mod request;
+/// Contains block request execution abstraction.
+pub mod stdio_executor;

src/block/request.rs

@@ -31,9 +31,12 @@ use vm_memory::{
     ByteValued, Bytes, GuestAddress, GuestAddressSpace, GuestMemory, GuestMemoryError,
 };
 
-const VIRTIO_BLK_T_IN: u32 = 0;
-const VIRTIO_BLK_T_OUT: u32 = 1;
-const VIRTIO_BLK_T_FLUSH: u32 = 4;
+/// Read request.
+pub const VIRTIO_BLK_T_IN: u32 = 0;
+/// Write request.
+pub const VIRTIO_BLK_T_OUT: u32 = 1;
+/// Flush request.
+pub const VIRTIO_BLK_T_FLUSH: u32 = 4;
 
 /// Block request parsing errors.
 #[derive(Debug)]
@@ -141,6 +144,11 @@ impl Request {
         self.status_addr
     }
 
+    /// Returns the total length of request data.
+    pub fn total_data_len(&self) -> u32 {
+        self.data.iter().map(|&x| x.1).sum()
+    }
+
     // Checks that a descriptor meets the minimal requirements for a valid status descriptor.
     fn check_status_desc<M: GuestMemory>(mem: &M, desc: Descriptor) -> Result<()> {
         // The status MUST always be writable.
@@ -264,6 +272,22 @@ mod tests {
         }
     }
 
+    impl Request {
+        pub fn new(
+            request_type: RequestType,
+            data: Vec<(GuestAddress, u32)>,
+            sector: u64,
+            status_addr: GuestAddress,
+        ) -> Self {
+            Request {
+                request_type,
+                data,
+                sector,
+                status_addr,
+            }
+        }
+    }
+
     // Helper method that writes a descriptor chain to a `GuestMemoryMmap` object and returns
     // the associated `DescriptorChain` object. `descs` represents a slice of `Descriptor` objects
     // which are used to populate the chain. This method ensures the next flags and values are
@@ -474,6 +498,7 @@ mod tests {
         };
         assert_eq!(request, expected_request);
         assert_eq!(request.status_addr(), GuestAddress(0x40_0000));
+        assert_eq!(request.total_data_len(), 0x100 + 0x200);
 
         // Request header with unsupported request type.
         let req_header = RequestHeader {