virtio/block: implement support for F_DISCARD

The F_DISCARD feature enables the guest to request us to free up sectors
on the backing storage. Since imago does most of the heavy lifting, we
just need to announce the feature and call to imago's "discard_to_any"
then needed.

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

Author: slp

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

@@ -247,6 +247,7 @@ impl Block {
         #[cfg(target_os = "macos")]
         let file_opts = file_opts.relaxed_sync(true);
         let file = ImagoFile::open_sync(file_opts)?;
+        let discard_alignment = file.discard_align();
 
         let disk_image = match disk_image_format {
             ImageType::Qcow2 => {
@@ -275,6 +276,7 @@ impl Block {
         let mut avail_features = (1u64 << VIRTIO_F_VERSION_1)
             | (1u64 << VIRTIO_BLK_F_FLUSH)
             | (1u64 << VIRTIO_BLK_F_SEG_MAX)
+            | (1u64 << VIRTIO_BLK_F_DISCARD)
             | (1u64 << VIRTIO_RING_F_EVENT_IDX);
 
         if is_disk_read_only {
@@ -290,6 +292,9 @@ impl Block {
             size_max: 0,
             // QUEUE_SIZE - 2
             seg_max: 254,
+            max_discard_sectors: u32::MAX,
+            max_discard_seg: 1,
+            discard_sector_alignment: discard_alignment as u32 / 512,
             ..Default::default()
         };
 

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

@@ -16,6 +16,7 @@ use vm_memory::{ByteValued, GuestMemoryMmap};
 #[allow(dead_code)]
 #[derive(Debug)]
 pub enum RequestError {
+    Discarding(io::Error),
     FlushingToDisk(io::Error),
     InvalidDataLength,
     ReadingFromDescriptor(io::Error),
@@ -39,10 +40,19 @@ pub struct RequestHeader {
     _reserved: u32,
     sector: u64,
 }
-
 // Safe because RequestHeader only contains plain data.
 unsafe impl ByteValued for RequestHeader {}
 
+#[derive(Copy, Clone, Default)]
+#[repr(C)]
+pub struct DiscardWriteData {
+    sector: u64,
+    num_sectors: u32,
+    flags: u32,
+}
+// Safe because DiscardWriteData only contains plain data.
+unsafe impl ByteValued for DiscardWriteData {}
+
 pub struct BlockWorker {
     queue: Queue,
     queue_evt: EventFd,
@@ -247,6 +257,21 @@ impl BlockWorker {
                     Ok(disk_id.len())
                 }
             }
+            VIRTIO_BLK_T_DISCARD => {
+                let discard_write_data: DiscardWriteData = reader
+                    .read_obj()
+                    .map_err(RequestError::ReadingFromDescriptor)?;
+                self.disk
+                    .file
+                    .lock()
+                    .unwrap()
+                    .discard_to_any(
+                        discard_write_data.sector * 512,
+                        discard_write_data.num_sectors as u64 * 512,
+                    )
+                    .map_err(RequestError::Discarding)?;
+                Ok(0)
+            }
             _ => Err(RequestError::UnknownRequest),
         }
     }