[RFC v2 4/5] virtiofs: perform DMA operations out of the spinlock

[Eugenio Pérez]

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 1124MiB/s to 1191MiB/s.

Signed-off-by: Eugenio Pérez <epere...@redhat.com>
---
 fs/fuse/virtio_fs.c | 40 ++++++++++++++++++++++++++++++++++++++--
 1 file changed, 38 insertions(+), 2 deletions(-)

diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
index 1344c5782a7c..e19c78f2480e 100644
--- a/fs/fuse/virtio_fs.c
+++ b/fs/fuse/virtio_fs.c
@@ -836,8 +836,19 @@ static void virtio_fs_requests_done_work(struct 
work_struct *work)
 
        /* End requests */
        list_for_each_entry_safe(req, next, &reqs, list) {
+               unsigned int total_sgs = req->out_sgs + req->in_sgs;
+
                list_del_init(&req->list);
 
+               for (unsigned int i = 0; i < total_sgs; ++i) {
+                       enum dma_data_direction dir = (i < req->out_sgs) ?
+                               DMA_TO_DEVICE : DMA_FROM_DEVICE;
+                       dma_unmap_page(vq->vdev->dev.parent,
+                                      sg_dma_address(&req->sg[i]),
+                                      sg_dma_len(&req->sg[i]), dir);
+
+               }
+
                /* blocking async request completes in a worker context */
                if (req->args->may_block) {
                        struct virtio_fs_req_work *w;
@@ -1426,6 +1437,24 @@ 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
+       for (i = 0; i < total_sgs; i++) {
+               struct page *page = sg_page(&req->sg[i]);
+               enum dma_data_direction dir = (i < req->out_sgs) ?
+                       DMA_TO_DEVICE : DMA_FROM_DEVICE;
+               dma_addr_t dma_addr = dma_map_page(vq->vdev->dev.parent, page,
+                                                  req->sg[i].offset, 
req->sg[i].length, dir);
+
+               if (dma_mapping_error(vq->vdev->dev.parent, dma_addr)) {
+                       ret = -ENOMEM;
+                       goto out;
+               }
+               sg_dma_address(&req->sg[i]) = dma_addr;
+               sg_dma_len(&req->sg[i]) = req->sg[i].length;
+       }
+
        spin_lock(&fsvq->lock);
 
        if (!fsvq->connected) {
@@ -1434,8 +1463,8 @@ 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;
@@ -1460,6 +1489,13 @@ static int virtio_fs_enqueue_req(struct virtio_fs_vq 
*fsvq,
                virtqueue_notify(vq);
 
 out:
+       for (unsigned int j = 0; ret && j < total_sgs; ++j) {
+               enum dma_data_direction dir = (j < req->out_sgs) ?
+                       DMA_TO_DEVICE : DMA_FROM_DEVICE;
+               dma_unmap_page(vq->vdev->dev.parent,
+                              sg_dma_address(&req->sg[j]),
+                              sg_dma_len(&req->sg[j]), dir);
+       }
        if (ret < 0 && req->argbuf) {
                kfree(req->argbuf);
                req->argbuf = NULL;
-- 
2.48.1