linux-kernel - [PATCH 1/1] virtio_ring: fix return code on DMA mapping fails

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Date:   Thu, 14 Nov 2019 13:46:46 +0100
From:   Halil Pasic <pasic@...ux.ibm.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Cc:     Halil Pasic <pasic@...ux.ibm.com>,
        Cornelia Huck <cohuck@...hat.com>, linux-s390@...r.kernel.org,
        Michael Mueller <mimu@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Christoph Hellwig <hch@....de>, Ram Pai <linuxram@...ibm.com>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        "Lendacky, Thomas" <Thomas.Lendacky@....com>,
        Andy Lutomirski <luto@...nel.org>
Subject: [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@...ux.ibm.com>
Tested-by: Michael Mueller <mimu@...ux.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