block: Add sparse operations capability query

Add capability query to DiskFile trait to check backend
support for sparse operations (punch hole, write zeroes,
discard). Only advertise VIRTIO_BLK_F_DISCARD and
VIRTIO_BLK_F_WRITE_ZEROES when the backend supports these
operations.

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

block/src/async_io.rs

@@ -78,6 +78,12 @@ pub trait DiskFile: Send {
         Err(DiskFileError::Unsupported)
     }
 
+    /// Indicates support for sparse operations (punch hole, write zeroes, discard).
+    /// Override to return true when supported.
+    fn supports_sparse_operations(&self) -> bool {
+        false
+    }
+
     /// Returns the file descriptor of the underlying disk image file.
     ///
     /// The file descriptor is supposed to be used for `fcntl()` calls but no

block/src/qcow_sync.rs

@@ -76,6 +76,10 @@ impl DiskFile for QcowDiskSync {
             .map_err(|e| DiskFileError::ResizeError(io::Error::other(e)))
     }
 
+    fn supports_sparse_operations(&self) -> bool {
+        true
+    }
+
     fn fd(&mut self) -> BorrowedDiskFd<'_> {
         BorrowedDiskFd::new(self.qcow_file.lock().unwrap().as_raw_fd())
     }

block/src/raw_async.rs

@@ -13,7 +13,7 @@ use vmm_sys_util::eventfd::EventFd;
 use crate::async_io::{
     AsyncIo, AsyncIoError, AsyncIoResult, BorrowedDiskFd, DiskFile, DiskFileError, DiskFileResult,
 };
-use crate::{BatchRequest, DiskTopology, RequestType};
+use crate::{BatchRequest, DiskTopology, RequestType, probe_sparse_support};
 
 pub struct RawFileDisk {
     file: File,
@@ -59,6 +59,10 @@ impl DiskFile for RawFileDisk {
         self.file.set_len(size).map_err(DiskFileError::ResizeError)
     }
 
+    fn supports_sparse_operations(&self) -> bool {
+        probe_sparse_support(&self.file)
+    }
+
     fn fd(&mut self) -> BorrowedDiskFd<'_> {
         BorrowedDiskFd::new(self.file.as_raw_fd())
     }

block/src/raw_async_aio.rs

@@ -13,10 +13,10 @@ use log::warn;
 use vmm_sys_util::aio;
 use vmm_sys_util::eventfd::EventFd;
 
-use crate::DiskTopology;
 use crate::async_io::{
     AsyncIo, AsyncIoError, AsyncIoResult, BorrowedDiskFd, DiskFile, DiskFileError, DiskFileResult,
 };
+use crate::{DiskTopology, probe_sparse_support};
 
 pub struct RawFileDiskAio {
     file: File,
@@ -58,6 +58,10 @@ impl DiskFile for RawFileDiskAio {
         }
     }
 
+    fn supports_sparse_operations(&self) -> bool {
+        probe_sparse_support(&self.file)
+    }
+
     fn fd(&mut self) -> BorrowedDiskFd<'_> {
         BorrowedDiskFd::new(self.file.as_raw_fd())
     }

block/src/raw_sync.rs

@@ -10,10 +10,10 @@ use std::os::unix::io::{AsRawFd, RawFd};
 use log::warn;
 use vmm_sys_util::eventfd::EventFd;
 
-use crate::DiskTopology;
 use crate::async_io::{
     AsyncIo, AsyncIoError, AsyncIoResult, BorrowedDiskFd, DiskFile, DiskFileError, DiskFileResult,
 };
+use crate::{DiskTopology, probe_sparse_support};
 
 pub struct RawFileDiskSync {
     file: File,
@@ -52,6 +52,10 @@ impl DiskFile for RawFileDiskSync {
         }
     }
 
+    fn supports_sparse_operations(&self) -> bool {
+        probe_sparse_support(&self.file)
+    }
+
     fn fd(&mut self) -> BorrowedDiskFd<'_> {
         BorrowedDiskFd::new(self.file.as_raw_fd())
     }

virtio-devices/src/block.rs

@@ -702,6 +702,13 @@ impl Block {
                     | (1u64 << VIRTIO_BLK_F_SEG_MAX)
                     | (1u64 << VIRTIO_RING_F_EVENT_IDX)
                     | (1u64 << VIRTIO_RING_F_INDIRECT_DESC);
+
+                // Only advertise discard/write zeroes if the backend supports sparse operations
+                if disk_image.supports_sparse_operations() {
+                    avail_features |=
+                        (1u64 << VIRTIO_BLK_F_DISCARD) | (1u64 << VIRTIO_BLK_F_WRITE_ZEROES);
+                }
+
                 if iommu {
                     avail_features |= 1u64 << VIRTIO_F_IOMMU_PLATFORM;
                 }