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 <avkrasnov@salutedevices.com>
---
 Changelog:
 v2 -> v3:
  * Comment about 'page_to_virt()' is updated. I don't remove R-b,
    as this change is quiet small I guess.
 v6 -> v7:
  * Move arrays '*sgs' and 'bufs' to 'virtio_vsock' instead of being
    local variables. This allows to save stack space in cases of too
    big MAX_SKB_FRAGS.
  * Add 'WARN_ON_ONCE()' for handling nonlinear skbs - it checks that
    linear part of such skb contains only header.
  * R-b tag removed due to updates above.

 net/vmw_vsock/virtio_transport.c | 54 +++++++++++++++++++++++++++-----
 1 file changed, 47 insertions(+), 7 deletions(-)

diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
index e95df847176b..8636477cf088 100644
--- a/net/vmw_vsock/virtio_transport.c
+++ b/net/vmw_vsock/virtio_transport.c
@@ -63,6 +63,10 @@ struct virtio_vsock {
 
 	u32 guest_cid;
 	bool seqpacket_allow;
+
+	/* +1 is for packet header. */
+	struct scatterlist *sgs[MAX_SKB_FRAGS + 1];
+	struct scatterlist bufs[MAX_SKB_FRAGS + 1];
 };
 
 static u32 virtio_transport_get_local_cid(void)
@@ -100,8 +104,9 @@ virtio_transport_send_pkt_work(struct work_struct *work)
 	vq = vsock->vqs[VSOCK_VQ_TX];
 
 	for (;;) {
-		struct scatterlist hdr, buf, *sgs[2];
 		int ret, in_sg = 0, out_sg = 0;
+		struct scatterlist **sgs;
+		struct scatterlist *bufs;
 		struct sk_buff *skb;
 		bool reply;
 
@@ -111,12 +116,47 @@ virtio_transport_send_pkt_work(struct work_struct *work)
 
 		virtio_transport_deliver_tap_pkt(skb);
 		reply = virtio_vsock_skb_reply(skb);
-
-		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;
+		sgs = vsock->sgs;
+		bufs = vsock->bufs;
+		sg_init_one(&bufs[out_sg], virtio_vsock_hdr(skb),
+			    sizeof(*virtio_vsock_hdr(skb)));
+		sgs[out_sg] = &bufs[out_sg];
+		out_sg++;
+
+		if (!skb_is_nonlinear(skb)) {
+			if (skb->len > 0) {
+				sg_init_one(&bufs[out_sg], skb->data, skb->len);
+				sgs[out_sg] = &bufs[out_sg];
+				out_sg++;
+			}
+		} else {
+			struct skb_shared_info *si;
+			int i;
+
+			/* If skb is nonlinear, then its buffer must contain
+			 * only header and nothing more. Data is stored in
+			 * the fragged part.
+			 */
+			WARN_ON_ONCE(skb_headroom(skb) != sizeof(*virtio_vsock_hdr(skb)));
+
+			si = skb_shinfo(skb);
+
+			for (i = 0; i < si->nr_frags; i++) {
+				skb_frag_t *skb_frag = &si->frags[i];
+				void *va;
+
+				/* We will use 'page_to_virt()' for the userspace page
+				 * here, because virtio or dma-mapping layers will call
+				 * 'virt_to_phys()' later to fill the buffer descriptor.
+				 * We don't touch memory at "virtual" address of this page.
+				 */
+				va = page_to_virt(skb_frag->bv_page);
+				sg_init_one(&bufs[out_sg],
+					    va + skb_frag->bv_offset,
+					    skb_frag->bv_len);
+				sgs[out_sg] = &bufs[out_sg];
+				out_sg++;
+			}
 		}
 
 		ret = virtqueue_add_sgs(vq, sgs, out_sg, in_sg, skb, GFP_KERNEL);
-- 
2.25.1

On Sun, Aug 27, 2023 at 11:54:34AM +0300, Arseniy Krasnov wrote:
>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 <avkrasnov@salutedevices.com>
>---
> Changelog:
> v2 -> v3:
>  * Comment about 'page_to_virt()' is updated. I don't remove R-b,
>    as this change is quiet small I guess.
> v6 -> v7:
>  * Move arrays '*sgs' and 'bufs' to 'virtio_vsock' instead of being
>    local variables. This allows to save stack space in cases of too
>    big MAX_SKB_FRAGS.
>  * Add 'WARN_ON_ONCE()' for handling nonlinear skbs - it checks that
>    linear part of such skb contains only header.
>  * R-b tag removed due to updates above.
>
> net/vmw_vsock/virtio_transport.c | 54 +++++++++++++++++++++++++++-----
> 1 file changed, 47 insertions(+), 7 deletions(-)
>
>diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
>index e95df847176b..8636477cf088 100644
>--- a/net/vmw_vsock/virtio_transport.c
>+++ b/net/vmw_vsock/virtio_transport.c
>@@ -63,6 +63,10 @@ struct virtio_vsock {
>
> 	u32 guest_cid;
> 	bool seqpacket_allow;
>+

I'd add a comment here specifying specifying what we need these fields
for and why we put them here (basically Paolo's suggestion).

>+	/* +1 is for packet header. */
>+	struct scatterlist *sgs[MAX_SKB_FRAGS + 1];
>+	struct scatterlist bufs[MAX_SKB_FRAGS + 1];
> };
>
> static u32 virtio_transport_get_local_cid(void)
>@@ -100,8 +104,9 @@ virtio_transport_send_pkt_work(struct work_struct *work)
> 	vq = vsock->vqs[VSOCK_VQ_TX];
>
> 	for (;;) {
>-		struct scatterlist hdr, buf, *sgs[2];
> 		int ret, in_sg = 0, out_sg = 0;
>+		struct scatterlist **sgs;
>+		struct scatterlist *bufs;
> 		struct sk_buff *skb;
> 		bool reply;
>
>@@ -111,12 +116,47 @@ virtio_transport_send_pkt_work(struct work_struct *work)
>
> 		virtio_transport_deliver_tap_pkt(skb);
> 		reply = virtio_vsock_skb_reply(skb);
>-
>-		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;
>+		sgs = vsock->sgs;
>+		bufs = vsock->bufs;
>+		sg_init_one(&bufs[out_sg], virtio_vsock_hdr(skb),
>+			    sizeof(*virtio_vsock_hdr(skb)));
>+		sgs[out_sg] = &bufs[out_sg];

IIUC `sgs[i]` always contains `&bufs[i]`.

Could we initialize it once when we allocate `struct virtio_vsock` in
`virtio_vsock_probe`?

Of course putting a comment in `struct virtio_vsock` about it.

Since we are using them only for out buffers, I'd also rename them in
out_sgs and out_bufs.

The rest LGTM.

Stefano

>+		out_sg++;
>+
>+		if (!skb_is_nonlinear(skb)) {
>+			if (skb->len > 0) {
>+				sg_init_one(&bufs[out_sg], skb->data, skb->len);
>+				sgs[out_sg] = &bufs[out_sg];
>+				out_sg++;
>+			}
>+		} else {
>+			struct skb_shared_info *si;
>+			int i;
>+
>+			/* If skb is nonlinear, then its buffer must contain
>+			 * only header and nothing more. Data is stored in
>+			 * the fragged part.
>+			 */
>+			WARN_ON_ONCE(skb_headroom(skb) != sizeof(*virtio_vsock_hdr(skb)));
>+
>+			si = skb_shinfo(skb);
>+
>+			for (i = 0; i < si->nr_frags; i++) {
>+				skb_frag_t *skb_frag = &si->frags[i];
>+				void *va;
>+
>+				/* We will use 'page_to_virt()' for the userspace page
>+				 * here, because virtio or dma-mapping layers will call
>+				 * 'virt_to_phys()' later to fill the buffer descriptor.
>+				 * We don't touch memory at "virtual" address of this page.
>+				 */
>+				va = page_to_virt(skb_frag->bv_page);
>+				sg_init_one(&bufs[out_sg],
>+					    va + skb_frag->bv_offset,
>+					    skb_frag->bv_len);
>+				sgs[out_sg] = &bufs[out_sg];
>+				out_sg++;
>+			}
> 		}
>
> 		ret = virtqueue_add_sgs(vq, sgs, out_sg, in_sg, skb, GFP_KERNEL);
>-- 
>2.25.1
>