Linux Virtualization - Nov 2019 - [PATCH 1/1] virtio_ring: fix return code on DMA mapping fails

Commit 780bc7903a32 ("virtio_ring: Support DMA APIs")  makes
virtqueue_add() return -EIO when we fail to map our I/O buffers. This is
a very realistic scenario for guests with encrypted memory, as swiotlb
may run out of space, depending on it's size and the I/O load.

The virtio-blk driver interprets -EIO form virtqueue_add() as an IO
error, despite the fact that swiotlb full is in absence of bugs a
recoverable condition.

Let us change the return code to -ENOMEM, and make the block layer
recover form these failures when virtio-blk encounters the condition
described above.

Fixes: 780bc7903a32 ("virtio_ring: Support DMA APIs")
Signed-off-by: Halil Pasic <pasic at linux.ibm.com>
Tested-by: Michael Mueller <mimu at linux.ibm.com>
---

Notes
====
* When out of descriptors (which might regarded as a similar out of
resources condition) virtio uses -ENOSPC, this however seems wrong,
as ENOSPC is defined as -ENOSPC. Thus I choose -ENOMEM over -ENOSPC.

* In virtio_queue_rq() in virtio_blk.c both -ENOMEM and -ENOSPC are
handled as BLK_STS_DEV_RESOURCE. Returning BLK_STS_RESOURCE however
seems more appropriate for dma mapping failed as we are talking about
a global, and not a device local resource. Both seem to do the trick.

* Mimu tested the patch with virtio-blk and virtio-net (thanks!). We
should look into how other virtio devices behave when DMA mapping fails.
---
 drivers/virtio/virtio_ring.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index a8041e451e9e..867c7ebd3f10 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -583,7 +583,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
 		kfree(desc);
 
 	END_USE(vq);
-	return -EIO;
+	return -ENOMEM;
 }
 
 static bool virtqueue_kick_prepare_split(struct virtqueue *_vq)
@@ -1085,7 +1085,7 @@ static int virtqueue_add_indirect_packed(struct
vring_virtqueue *vq,
 	kfree(desc);
 
 	END_USE(vq);
-	return -EIO;
+	return -ENOMEM;
 }
 
 static inline int virtqueue_add_packed(struct virtqueue *_vq,
-- 
2.17.1