virtio-blk: blockdev-mirroring for live storage migration

block/Cargo.toml

@@ -13,6 +13,7 @@ io_uring = ["dep:io-uring"]
 bitflags = { workspace = true }
 byteorder = { workspace = true }
 crc-any = "2.5.0"
+event_monitor = { path = "../event_monitor" }
 flate2 = "1.1"
 io-uring = { version = "0.7.12", optional = true }
 libc = { workspace = true }

block/src/async_io.rs

@@ -2,6 +2,7 @@
 //
 // SPDX-License-Identifier: Apache-2.0 AND BSD-3-Clause
 
+use std::io;
 use std::marker::PhantomData;
 use std::os::fd::{AsRawFd, OwnedFd, RawFd};
 
@@ -58,6 +59,28 @@ impl AsRawFd for BorrowedDiskFd<'_> {
     }
 }
 
+/// Returns true when `left` and `right` refer to the same file.
+pub fn is_same_file(left: BorrowedDiskFd, right: BorrowedDiskFd) -> bool {
+    matches!(
+        (file_identity(left), file_identity(right)),
+        (Ok(left), Ok(right)) if left == right
+    )
+}
+
+/// Returns the `(device ID, inode number)` identity of the file behind `fd`.
+fn file_identity(fd: BorrowedDiskFd) -> io::Result<(u64, u64)> {
+    let mut stat = std::mem::MaybeUninit::<libc::stat>::zeroed();
+    // SAFETY: `stat` points to writable memory for one `libc::stat`.
+    // `fstat` does not close or take ownership of the borrowed file descriptor.
+    let ret = unsafe { libc::fstat(fd.as_raw_fd(), stat.as_mut_ptr()) };
+    if ret != 0 {
+        return Err(io::Error::last_os_error());
+    }
+    // SAFETY: the zeroed buffer is a valid `libc::stat`.
+    let stat = unsafe { stat.assume_init() };
+    Ok((stat.st_dev, stat.st_ino))
+}
+
 #[derive(Error, Debug)]
 pub enum AsyncIoError {
     /// Failed vectored reading from file.

block/src/disk_file.rs

@@ -36,7 +36,7 @@
 use std::fmt::Debug;
 
 use crate::async_io::{AsyncIo, BorrowedDiskFd};
-use crate::{BlockResult, DiskTopology};
+use crate::{BlockError, BlockErrorKind, BlockResult, DiskTopology};
 
 /// Reported capacity of a disk image.
 pub trait DiskSize: Send + Debug {
@@ -96,7 +96,12 @@ pub trait Resizable: Send + Debug {
 /// Every disk format implements `DiskSize` and `Geometry`.
 /// `Sync` is required so that `Arc<dyn DiskFile>` can be shared
 /// across threads for concurrent readonly access.
-pub trait DiskFile: DiskSize + Geometry + Sync {}
+pub trait DiskFile: DiskSize + Geometry + Sync {
+    /// Returns an error if this disk image cannot participate in block mirroring.
+    fn supports_mirroring(&self) -> BlockResult<()> {
+        Err(BlockError::from_kind(BlockErrorKind::UnsupportedFeature))
+    }
+}
 
 /// Full capability disk file trait.
 ///

block/src/lib.rs

@@ -20,6 +20,7 @@ pub mod fixed_vhd;
 pub mod fixed_vhd_async;
 pub mod fixed_vhd_disk;
 pub mod fixed_vhd_sync;
+pub mod mirror;
 pub mod qcow;
 #[cfg(feature = "io_uring")]
 pub(crate) mod qcow_async;