On Tue, Jun 27, 2023 at 07:39:41AM +0300, Arseniy Krasnov wrote:

On 26.06.2023 18:36, Stefano Garzarella wrote:

quoted

On Sat, Jun 03, 2023 at 11:49:25PM +0300, Arseniy Krasnov wrote:

quoted

For non-linear skb use its pages from fragment array as buffers in
virtio tx queue. These pages are already pinned by 'get_user_pages()'
during such skb creation.

Signed-off-by: Arseniy Krasnov <redacted>
---
net/vmw_vsock/virtio_transport.c | 37 ++++++++++++++++++++++++++------
1 file changed, 31 insertions(+), 6 deletions(-)

diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
index e95df847176b..6053d8341091 100644
--- a/net/vmw_vsock/virtio_transport.c
+++ b/net/vmw_vsock/virtio_transport.c

@@ -100,7 +100,9 @@ virtio_transport_send_pkt_work(struct work_struct *work)

    vq = vsock->vqs[VSOCK_VQ_TX];

    for (;;) {
-        struct scatterlist hdr, buf, *sgs[2];
+        /* +1 is for packet header. */
+        struct scatterlist *sgs[MAX_SKB_FRAGS + 1];
+        struct scatterlist bufs[MAX_SKB_FRAGS + 1];
        int ret, in_sg = 0, out_sg = 0;
        struct sk_buff *skb;
        bool reply;

@@ -111,12 +113,35 @@ virtio_transport_send_pkt_work(struct work_struct *work)

        virtio_transport_deliver_tap_pkt(skb);
        reply = virtio_vsock_skb_reply(skb);
+        sg_init_one(&bufs[0], virtio_vsock_hdr(skb), sizeof(*virtio_vsock_hdr(skb)));
+        sgs[out_sg++] = &bufs[0];

Can we use out_sg also to index bufs (here and in the rest of the code)?

E.g.

        sg_init_one(&bufs[out_sg], ...)
        sgs[out_sg] = &bufs[out_sg];
        ++out_sg;

        ...
            if (skb->len > 0) {
                sg_init_one(&bufs[out_sg], skb->data, skb->len);
                sgs[out_sg] = &bufs[out_sg];
                ++out_sg;
            }

        etc...

quoted

+

For readability, I would move the smaller branch above:

        if (!skb_is_nonlinear(skb)) {
            // small block
            ...
        } else {
            // big block
            ...
        }

quoted

+        if (skb_is_nonlinear(skb)) {
+            struct skb_shared_info *si;
+            int i;
+
+            si = skb_shinfo(skb);
+
+            for (i = 0; i < si->nr_frags; i++) {
+                skb_frag_t *skb_frag = &si->frags[i];
+                void *va = page_to_virt(skb_frag->bv_page);
+
+                /* We will use 'page_to_virt()' for userspace page here,
+                 * because virtio layer will call 'virt_to_phys()' later
+                 * to fill buffer descriptor. We don't touch memory at
+                 * "virtual" address of this page.
+                 */
+                sg_init_one(&bufs[i + 1],
+                        va + skb_frag->bv_offset,
+                        skb_frag->bv_len);
+                sgs[out_sg++] = &bufs[i + 1];
+            }
+        } else {
+            if (skb->len > 0) {

Should we do the same check (skb->len > 0) for nonlinear skb as well?
Or do the nonlinear ones necessarily have len > 0?

Yes, non-linear skb always has 'data_len' > 0, e.g. such skbs always have some
data in it.

Okay, makes sense ;-)

Thanks,
Stefano

Thanks, Arseniy

quoted

quoted

+                sg_init_one(&bufs[1], skb->data, skb->len);
+                sgs[out_sg++] = &bufs[1];
+            }

   ^
Blank line that we can remove.

Stefano

quoted

-        sg_init_one(&hdr, virtio_vsock_hdr(skb), sizeof(*virtio_vsock_hdr(skb)));
-        sgs[out_sg++] = &hdr;
-        if (skb->len > 0) {
-            sg_init_one(&buf, skb->data, skb->len);
-            sgs[out_sg++] = &buf;
        }

        ret = virtqueue_add_sgs(vq, sgs, out_sg, in_sg, skb, GFP_KERNEL);
-- 
2.25.1