disk: switch from WriteZeroes to WriteZeroesAt

This eliminates an extra seek per guest write zeroes request.

Additionally, it allows us to stop depending on the file cursor and pass
the offset directly, making multi-queue implementation easier.

BUG=chromium:858815
TEST=Boot Termina in crosvm

Change-Id: I8b15a39752a1b68597a2b1e1fd72382a484a3cb2
Signed-off-by: Daniel Verkamp <dverkamp@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/1913521
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Dylan Reid <dgreid@chromium.org>

devices/src/virtio/block.rs

@@ -140,10 +140,6 @@ enum ExecuteError {
         sector: u64,
         desc_error: io::Error,
     },
-    Seek {
-        ioerr: io::Error,
-        sector: u64,
-    },
     TimerFd(SysError),
     WriteIo {
         length: usize,
@@ -182,7 +178,6 @@ impl Display for ExecuteError {
                 "io error reading {} bytes from sector {}: {}",
                 length, sector, desc_error,
             ),
-            Seek { ioerr, sector } => write!(f, "failed to seek to sector {}: {}", sector, ioerr),
             TimerFd(e) => write!(f, "{}", e),
             WriteIo {
                 length,
@@ -229,7 +224,6 @@ impl ExecuteError {
             ExecuteError::WriteStatus(_) => VIRTIO_BLK_S_IOERR,
             ExecuteError::Flush(_) => VIRTIO_BLK_S_IOERR,
             ExecuteError::ReadIo { .. } => VIRTIO_BLK_S_IOERR,
-            ExecuteError::Seek { .. } => VIRTIO_BLK_S_IOERR,
             ExecuteError::TimerFd(_) => VIRTIO_BLK_S_IOERR,
             ExecuteError::WriteIo { .. } => VIRTIO_BLK_S_IOERR,
             ExecuteError::DiscardWriteZeroes { .. } => VIRTIO_BLK_S_IOERR,
@@ -661,16 +655,13 @@ impl Block {
                         // FALLOC_FL_PUNCH_HOLE, ignore punch_hole errors.
                         let _ = disk.punch_hole(offset, length);
                     } else {
-                        disk.seek(SeekFrom::Start(offset))
-                            .map_err(|e| ExecuteError::Seek { ioerr: e, sector })?;
-                        disk.write_zeroes_all(length as usize).map_err(|e| {
-                            ExecuteError::DiscardWriteZeroes {
+                        disk.write_zeroes_all_at(offset, length as usize)
+                            .map_err(|e| ExecuteError::DiscardWriteZeroes {
                                 ioerr: Some(e),
                                 sector,
                                 num_sectors,
                                 flags,
-                            }
-                        })?;
+                            })?;
                     }
                 }
             }

disk/src/composite.rs

@@ -14,7 +14,7 @@ use crate::{create_disk_file, DiskFile, ImageType};
 use data_model::VolatileSlice;
 use protos::cdisk_spec;
 use remain::sorted;
-use sys_util::{AsRawFds, FileReadWriteAtVolatile, FileSetLen, FileSync, PunchHole, WriteZeroes};
+use sys_util::{AsRawFds, FileReadWriteAtVolatile, FileSetLen, FileSync, PunchHole, WriteZeroesAt};
 
 #[sorted]
 #[derive(Debug)]
@@ -300,22 +300,17 @@ impl Seek for CompositeDiskFile {
     }
 }
 
-impl WriteZeroes for CompositeDiskFile {
-    fn write_zeroes(&mut self, length: usize) -> io::Result<usize> {
-        let cursor_location = self.cursor_location;
+impl WriteZeroesAt for CompositeDiskFile {
+    fn write_zeroes_at(&mut self, offset: u64, length: usize) -> io::Result<usize> {
+        let cursor_location = offset;
         let disk = self.disk_at_offset(cursor_location)?;
-        disk.file
-            .seek(SeekFrom::Start(cursor_location - disk.offset))?;
+        let offset_within_disk = cursor_location - disk.offset;
         let new_length = if cursor_location + length as u64 > disk.offset + disk.length {
             (disk.offset + disk.length - cursor_location) as usize
         } else {
             length
         };
-        let result = disk.file.write_zeroes(new_length);
-        if let Ok(size) = result {
-            self.cursor_location += size as u64;
-        }
-        result
+        disk.file.write_zeroes_at(offset_within_disk, new_length)
     }
 }
 
@@ -524,10 +519,11 @@ mod tests {
         composite
             .write_all_at_volatile(input_volatile_memory.get_slice(0, 300).unwrap(), 0)
             .unwrap();
-        composite.seek(SeekFrom::Start(50)).unwrap();
         let mut zeroes_written = 0;
         while zeroes_written < 200 {
-            zeroes_written += composite.write_zeroes(200 - zeroes_written).unwrap();
+            zeroes_written += composite
+                .write_zeroes_at(50 + zeroes_written as u64, 200 - zeroes_written)
+                .unwrap();
         }
         let mut output_memory = [0u8; 300];
         let output_volatile_memory = &mut output_memory[..];

disk/src/disk.rs

@@ -11,7 +11,7 @@ use libc::EINVAL;
 use qcow::{QcowFile, QCOW_MAGIC};
 use remain::sorted;
 use sys_util::{
-    AsRawFds, FileReadWriteAtVolatile, FileSetLen, FileSync, PunchHole, SeekHole, WriteZeroes,
+    AsRawFds, FileReadWriteAtVolatile, FileSetLen, FileSync, PunchHole, SeekHole, WriteZeroesAt,
 };
 
 #[cfg(feature = "composite-disk")]
@@ -45,7 +45,7 @@ pub trait DiskFile:
     + FileReadWriteAtVolatile
     + PunchHole
     + Seek
-    + WriteZeroes
+    + WriteZeroesAt
     + Send
     + AsRawFds
 {
@@ -56,7 +56,7 @@ impl<
             + PunchHole
             + FileReadWriteAtVolatile
             + Seek
-            + WriteZeroes
+            + WriteZeroesAt
             + Send
             + AsRawFds,
     > DiskFile for D