block: qcow: Move RawBacking to qcow/backing module

Move the raw backing file reader into the new qcow/backing module
so it can be shared between qcow_sync and the upcoming qcow_async
backend.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

block/src/qcow/backing.rs

@@ -0,0 +1,40 @@
+// Copyright © 2021 Intel Corporation
+//
+// Copyright 2026 The Cloud Hypervisor Authors. All rights reserved.
+//
+// SPDX-License-Identifier: Apache-2.0 AND BSD-3-Clause
+
+//! Thread safe backing file readers for QCOW2 images.
+
+use std::io;
+use std::os::fd::{AsRawFd, OwnedFd};
+
+use crate::qcow::metadata::BackingRead;
+use crate::qcow_common::pread_exact;
+
+/// Raw backing file using pread64 on a duplicated fd.
+pub(crate) struct RawBacking {
+    pub(crate) fd: OwnedFd,
+    pub(crate) virtual_size: u64,
+}
+
+// SAFETY: The only I/O operation is pread64 which is position independent
+// and safe for concurrent use from multiple threads.
+unsafe impl Sync for RawBacking {}
+
+impl BackingRead for RawBacking {
+    fn read_at(&self, address: u64, buf: &mut [u8]) -> io::Result<()> {
+        if address >= self.virtual_size {
+            buf.fill(0);
+            return Ok(());
+        }
+        let available = (self.virtual_size - address) as usize;
+        if available >= buf.len() {
+            pread_exact(self.fd.as_raw_fd(), buf, address)
+        } else {
+            pread_exact(self.fd.as_raw_fd(), &mut buf[..available], address)?;
+            buf[available..].fill(0);
+            Ok(())
+        }
+    }
+}

block/src/qcow/mod.rs

@@ -4,6 +4,7 @@
 //
 // SPDX-License-Identifier: Apache-2.0 AND BSD-3-Clause
 
+pub(crate) mod backing;
 mod decoder;
 mod header;
 pub(crate) mod metadata;

block/src/qcow_sync.rs

@@ -15,6 +15,7 @@ use vmm_sys_util::write_zeroes::{PunchHole, WriteZeroesAt};
 use crate::async_io::{AsyncIo, AsyncIoError, AsyncIoResult, BorrowedDiskFd, DiskFileError};
 use crate::disk_file;
 use crate::error::{BlockError, BlockErrorKind, BlockResult, ErrorOp};
+use crate::qcow::backing::RawBacking;
 use crate::qcow::metadata::{
     BackingRead, ClusterReadMapping, ClusterWriteMapping, DeallocAction, QcowMetadata,
 };
@@ -26,33 +27,6 @@ use crate::qcow_common::{
     gather_from_iovecs, pread_exact, pwrite_all, scatter_to_iovecs, zero_fill_iovecs,
 };
 
-/// Raw backing file using pread64 on a duplicated fd.
-struct RawBacking {
-    fd: OwnedFd,
-    virtual_size: u64,
-}
-
-// SAFETY: The only I/O operation is pread64 which is position independent
-// and safe for concurrent use from multiple threads.
-unsafe impl Sync for RawBacking {}
-
-impl BackingRead for RawBacking {
-    fn read_at(&self, address: u64, buf: &mut [u8]) -> io::Result<()> {
-        if address >= self.virtual_size {
-            buf.fill(0);
-            return Ok(());
-        }
-        let available = (self.virtual_size - address) as usize;
-        if available >= buf.len() {
-            pread_exact(self.fd.as_raw_fd(), buf, address)
-        } else {
-            pread_exact(self.fd.as_raw_fd(), &mut buf[..available], address)?;
-            buf[available..].fill(0);
-            Ok(())
-        }
-    }
-}
-
 /// QCOW2 backing file with RwLock metadata and pread64 data reads.
 ///
 /// Read only because backing files never receive writes. Nested backing