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Date:   Thu, 6 Jul 2023 18:51:01 +0200
From:   Stefano Garzarella <sgarzare@redhat.com>
To:     Arseniy Krasnov <AVKrasnov@sberdevices.ru>
Cc:     Stefan Hajnoczi <stefanha@redhat.com>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        Bobby Eshleman <bobby.eshleman@bytedance.com>,
        kvm@vger.kernel.org, virtualization@lists.linux-foundation.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel@sberdevices.ru, oxffffaa@gmail.com
Subject: Re: [RFC PATCH v5 03/17] vsock/virtio: support to send non-linear skb
Message-ID: <joml4wolu5r2wpetvbfezu6buwfnfntu5okxjacfgdsp7uaebk@onkmwg7r7fod>
References: <20230701063947.3422088-1-AVKrasnov@sberdevices.ru>
 <20230701063947.3422088-4-AVKrasnov@sberdevices.ru>
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In-Reply-To: <20230701063947.3422088-4-AVKrasnov@sberdevices.ru>
Precedence: bulk

On Sat, Jul 01, 2023 at 09:39:33AM +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@sberdevices.ru>
>---
> Changelog:
> v4 -> v5:
>  * Use 'out_sgs' variable to index 'bufs', not only 'sgs'.
>  * Move smaller branch above, see 'if (!skb_is_nonlinear(skb)').
>  * Remove blank line.
>  * R-b from Bobby Eshleman removed due to patch update.
>
> net/vmw_vsock/virtio_transport.c | 40 +++++++++++++++++++++++++++-----
> 1 file changed, 34 insertions(+), 6 deletions(-)

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>

>
>diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
>index e95df847176b..6cbb45bb12d2 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 +113,38 @@ 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[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;
>+
>+			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);
>
>-		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;
>+				/* 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[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
>