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Date:   Tue, 19 Nov 2019 12:10:22 +0100
From:   Halil Pasic <pasic@linux.ibm.com>
To:     "Michael S. Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org
Cc:     Cornelia Huck <cohuck@redhat.com>, linux-s390@vger.kernel.org,
        Michael Mueller <mimu@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Janosch Frank <frankja@linux.ibm.com>,
        Christoph Hellwig <hch@lst.de>, Ram Pai <linuxram@us.ibm.com>,
        Thiago Jung Bauermann <bauerman@linux.ibm.com>,
        "Lendacky, Thomas" <Thomas.Lendacky@amd.com>,
        Andy Lutomirski <luto@kernel.org>
Subject: Re: [PATCH 1/1] virtio_ring: fix return code on DMA mapping fails
In-Reply-To: <20191114124646.74790-1-pasic@linux.ibm.com>
References: <20191114124646.74790-1-pasic@linux.ibm.com>
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On Thu, 14 Nov 2019 13:46:46 +0100
Halil Pasic <pasic@linux.ibm.com> wrote:

> 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@linux.ibm.com>
> Tested-by: Michael Mueller <mimu@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,