block: Add supports_zero_flag trait method

Add supports_zero_flag() to DiskFile trait to indicate whether a disk
format can mark clusters/blocks as reading zeros without deallocating
storage.

QCOW2 supports this via the zero flag in L2 entries. VHDX also has
PAYLOAD_BLOCK_ZERO state for this, though it's not yet implemented in
cloud-hypervisor.

This enables DISCARD to be advertised even with sparse=false for formats
with zero-flag support, since they can mark regions as zeros (keeps
storage allocated) instead of requiring full deallocation.

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

block/src/async_io.rs

@@ -84,6 +84,12 @@ pub trait DiskFile: Send {
         false
     }
 
+    /// Indicates support for zero flag optimization in WRITE_ZEROES. Override
+    /// to return true when supported.
+    fn supports_zero_flag(&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

@@ -80,6 +80,10 @@ impl DiskFile for QcowDiskSync {
         true
     }
 
+    fn supports_zero_flag(&self) -> bool {
+        true
+    }
+
     fn fd(&mut self) -> BorrowedDiskFd<'_> {
         BorrowedDiskFd::new(self.qcow_file.lock().unwrap().as_raw_fd())
     }

virtio-devices/src/block.rs

@@ -672,7 +672,7 @@ impl Block {
         exit_evt: EventFd,
         state: Option<BlockState>,
         queue_affinity: BTreeMap<u16, Vec<usize>>,
-        _sparse: bool,
+        sparse: bool,
     ) -> io::Result<Self> {
         let (disk_nsectors, avail_features, acked_features, config, paused) =
             if let Some(state) = state {
@@ -704,10 +704,17 @@ impl Block {
                     | (1u64 << VIRTIO_RING_F_EVENT_IDX)
                     | (1u64 << VIRTIO_RING_F_INDIRECT_DESC);
 
-                // Only advertise discard/write zeroes if the backend supports sparse operations
+                // When backend supports sparse operations:
+                // - Always advertise WRITE_ZEROES
+                // - Advertise DISCARD only if sparse=true OR format supports marking
+                //   clusters as zero without deallocating
                 if disk_image.supports_sparse_operations() {
-                    avail_features |=
-                        (1u64 << VIRTIO_BLK_F_DISCARD) | (1u64 << VIRTIO_BLK_F_WRITE_ZEROES);
+                    avail_features |= 1u64 << VIRTIO_BLK_F_WRITE_ZEROES;
+                    if sparse || disk_image.supports_zero_flag() {
+                        avail_features |= 1u64 << VIRTIO_BLK_F_DISCARD;
+                    }
+                } else if sparse {
+                    warn!("sparse=on requested but backend does not support sparse operations");
                 }
 
                 if iommu {