[RFC 5/5] virtiofs: perform DMA operations out of the spinlock

["Eugenio Pérez" <eperezma@redhat.com>]

This is useful for some setups like swiotlb or VDUSE where the DMA
operations are expensive and/or need to be performed with a write lock.

After applying this patch, fio read test goes from 1201MiB/s to 1211MiB/s.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
---
 drivers/virtio/virtio_ring.c |  2 ++
 fs/fuse/virtio_fs.c          | 25 +++++++++++++++++++++++--
 2 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index e49912fa77c5..eb22bfcb9100 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -580,6 +580,7 @@ int virtqueue_map_sgs(struct virtqueue *_vq,
 				goto unmap_release;
 
 			sg_dma_address(sg) = addr;
+			sg_dma_len(sg) = sg->length;
 			mapped_sg++;
 		}
 	}
@@ -592,6 +593,7 @@ int virtqueue_map_sgs(struct virtqueue *_vq,
 				goto unmap_release;
 
 			sg_dma_address(sg) = addr;
+			sg_dma_len(sg) = sg->length;
 			mapped_sg++;
 		}
 	}
diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
index 1344c5782a7c..2b558b05d0f8 100644
--- a/fs/fuse/virtio_fs.c
+++ b/fs/fuse/virtio_fs.c
@@ -836,8 +836,21 @@ static void virtio_fs_requests_done_work(struct work_struct *work)
 
 	/* End requests */
 	list_for_each_entry_safe(req, next, &reqs, list) {
+		struct scatterlist *stack_sgs[6];
+		struct scatterlist **sgs = stack_sgs;
+		unsigned int total_sgs = req->out_sgs + req->in_sgs;
+
 		list_del_init(&req->list);
 
+		/* TODO replace magic 6 by a macro */
+		if (total_sgs > 6)
+			sgs = kmalloc_array(total_sgs, sizeof(sgs[0]), GFP_ATOMIC);
+
+		for (unsigned int i = 0; i < total_sgs; ++i)
+			sgs[i] = &req->sg[i];
+
+		virtqueue_unmap_sgs(vq, sgs, req->out_sgs, req->in_sgs);
+
 		/* blocking async request completes in a worker context */
 		if (req->args->may_block) {
 			struct virtio_fs_req_work *w;
@@ -850,6 +863,9 @@ static void virtio_fs_requests_done_work(struct work_struct *work)
 		} else {
 			virtio_fs_request_complete(req, fsvq);
 		}
+
+		if (sgs != stack_sgs)
+			kfree(sgs);
 	}
 
 	/* Try to push previously queued requests, as the queue might no longer be full */
@@ -1426,6 +1442,11 @@ static int virtio_fs_enqueue_req(struct virtio_fs_vq *fsvq,
 		sgs[i] = &req->sg[i];
 	WARN_ON(req->out_sgs + req->in_sgs != total_sgs);
 
+	// TODO can we change this ptr out of the lock?
+	vq = fsvq->vq;
+	// TODO handle this and following errors
+	ret = virtqueue_map_sgs(vq, sgs, req->out_sgs, req->in_sgs);
+	BUG_ON(ret < 0);
 	spin_lock(&fsvq->lock);
 
 	if (!fsvq->connected) {
@@ -1434,8 +1455,8 @@ static int virtio_fs_enqueue_req(struct virtio_fs_vq *fsvq,
 		goto out;
 	}
 
-	vq = fsvq->vq;
-	ret = virtqueue_add_sgs(vq, sgs, req->out_sgs, req->in_sgs, req, GFP_ATOMIC);
+	ret = virtqueue_add_sgs_premapped(vq, sgs, req->out_sgs,
+					  req->in_sgs, req, GFP_ATOMIC);
 	if (ret < 0) {
 		spin_unlock(&fsvq->lock);
 		goto out;
-- 
2.48.1