From 3d786624988ed50de1e53b616e887b0eec3c605d Mon Sep 17 00:00:00 2001
From: Gauthier Jolly <contact@gjolly.fr>
Date: Wed, 28 May 2025 21:52:10 +0200
Subject: [PATCH] block: virtio-blk: report IO errors to the guest

Instead of exiting on IO errors, report the errors to the guest with
VIRTIO_BLK_S_IOERR. For example, the guest kernel will log something
similar to this if the nbd behind /dev/vdc is unexpectedly disconnected:

[  166.033957] I/O error, dev vdc, sector 264 op 0x1:(WRITE) flags 0x9800 phys_seg 1 prio class 2
[  166.035083] Aborting journal on device vdc-8.
[  166.037307] Buffer I/O error on dev vdc, logical block 9, lost sync page write
[  166.038471] JBD2: I/O error when updating journal superblock for vdc-8.
[...]
[  174.234470] EXT4-fs (vdc): I/O error while writing superblock

In case the rootfs is not located on the affected block device, this
will not crash the guest.

Fixes: #6995

Signed-off-by: Gauthier Jolly <contact@gjolly.fr>
---
 virtio-devices/src/block.rs | 33 ++++++++++++++++++++-------------
 1 file changed, 20 insertions(+), 13 deletions(-)

diff --git a/virtio-devices/src/block.rs b/virtio-devices/src/block.rs
index fc99649da..1589cda02 100644
--- a/virtio-devices/src/block.rs
+++ b/virtio-devices/src/block.rs
@@ -212,22 +212,29 @@ impl BlockEpollHandler {
 
             request.set_writeback(self.writeback.load(Ordering::Acquire));
 
-            if request
-                .execute_async(
-                    desc_chain.memory(),
-                    self.disk_nsectors,
-                    self.disk_image.as_mut(),
-                    &self.serial,
-                    desc_chain.head_index() as u64,
-                )
-                .map_err(Error::RequestExecuting)?
-            {
+            let result = request.execute_async(
+                desc_chain.memory(),
+                self.disk_nsectors,
+                self.disk_image.as_mut(),
+                &self.serial,
+                desc_chain.head_index() as u64,
+            );
+
+            if let Ok(true) = result {
                 self.inflight_requests
                     .push_back((desc_chain.head_index(), request));
             } else {
+                let status = match result {
+                    Ok(_) => VIRTIO_BLK_S_OK,
+                    Err(e) => {
+                        warn!("Request failed: {:x?} {:?}", request, e);
+                        VIRTIO_BLK_S_IOERR
+                    }
+                };
+
                 desc_chain
                     .memory()
-                    .write_obj(VIRTIO_BLK_S_OK as u8, request.status_addr)
+                    .write_obj(status as u8, request.status_addr)
                     .map_err(Error::RequestStatus)?;
 
                 // If no asynchronous operation has been submitted, we can
@@ -389,12 +396,12 @@ impl BlockEpollHandler {
 
                 (VIRTIO_BLK_S_OK as u8, result as u32)
             } else {
-                error!(
+                warn!(
                     "Request failed: {:x?} {:?}",
                     request,
                     io::Error::from_raw_os_error(-result)
                 );
-                return Err(Error::AsyncRequestFailure);
+                (VIRTIO_BLK_S_IOERR as u8, 0)
             };
 
             mem.write_obj(status, request.status_addr)