virtio/block: implement support for F_WRITE_ZEROES

The F_WRITE_ZEROES feature enables the guest to instruct the host to
write zeroes to the disk image without actually having to copy any data
around. Since imago does the heavy lifting for us, we just need to
announce the feature and call to the right method when needed.

Signed-off-by: Sergio Lopez <[email protected]>

Author: slp

src/devices/src/virtio/block/device.rs

@@ -277,6 +277,7 @@ impl Block {
             | (1u64 << VIRTIO_BLK_F_FLUSH)
             | (1u64 << VIRTIO_BLK_F_SEG_MAX)
             | (1u64 << VIRTIO_BLK_F_DISCARD)
+            | (1u64 << VIRTIO_BLK_F_WRITE_ZEROES)
             | (1u64 << VIRTIO_RING_F_EVENT_IDX);
 
         if is_disk_read_only {
@@ -295,6 +296,9 @@ impl Block {
             max_discard_sectors: u32::MAX,
             max_discard_seg: 1,
             discard_sector_alignment: discard_alignment as u32 / 512,
+            max_write_zeroes_sectors: u32::MAX,
+            max_write_zeroes_seg: 1,
+            write_zeroes_may_unmap: 1,
             ..Default::default()
         };
 

src/devices/src/virtio/block/worker.rs

@@ -17,10 +17,12 @@ use vm_memory::{ByteValued, GuestMemoryMmap};
 #[derive(Debug)]
 pub enum RequestError {
     Discarding(io::Error),
+    DiscardingToZero(io::Error),
     FlushingToDisk(io::Error),
     InvalidDataLength,
     ReadingFromDescriptor(io::Error),
     WritingToDescriptor(io::Error),
+    WritingZeroes(io::Error),
     UnknownRequest,
 }
 
@@ -272,6 +274,34 @@ impl BlockWorker {
                     .map_err(RequestError::Discarding)?;
                 Ok(0)
             }
+            VIRTIO_BLK_T_WRITE_ZEROES => {
+                let discard_write_data: DiscardWriteData = reader
+                    .read_obj()
+                    .map_err(RequestError::ReadingFromDescriptor)?;
+                let unmap = (discard_write_data.flags & VIRTIO_BLK_WRITE_ZEROES_FLAG_UNMAP) != 0;
+                if unmap {
+                    self.disk
+                        .file
+                        .lock()
+                        .unwrap()
+                        .discard_to_zero(
+                            discard_write_data.sector * 512,
+                            discard_write_data.num_sectors as u64 * 512,
+                        )
+                        .map_err(RequestError::DiscardingToZero)?;
+                } else {
+                    self.disk
+                        .file
+                        .lock()
+                        .unwrap()
+                        .write_zeroes(
+                            discard_write_data.sector * 512,
+                            discard_write_data.num_sectors as u64 * 512,
+                        )
+                        .map_err(RequestError::WritingZeroes)?;
+                }
+                Ok(0)
+            }
             _ => Err(RequestError::UnknownRequest),
         }
     }