linux-kernel - Re: [RFC PATCH v1 05/12] vsock/virtio: non-linear skb support

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Date:   Thu, 16 Feb 2023 15:18:56 +0100
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Arseniy Krasnov <AVKrasnov@...rdevices.ru>
Cc:     Stefan Hajnoczi <stefanha@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Krasnov Arseniy <oxffffaa@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        kernel <kernel@...rdevices.ru>
Subject: Re: [RFC PATCH v1 05/12] vsock/virtio: non-linear skb support

On Mon, Feb 06, 2023 at 06:58:24AM +0000, Arseniy Krasnov wrote:
>Use pages of non-linear skb as buffers in virtio tx queue.
>
>Signed-off-by: Arseniy Krasnov <AVKrasnov@...rdevices.ru>
>---
> net/vmw_vsock/virtio_transport.c | 31 +++++++++++++++++++++++++------
> 1 file changed, 25 insertions(+), 6 deletions(-)
>
>diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
>index 28b5a8e8e094..b8a7d6dc9f46 100644
>--- a/net/vmw_vsock/virtio_transport.c
>+++ b/net/vmw_vsock/virtio_transport.c
>@@ -100,7 +100,8 @@ virtio_transport_send_pkt_work(struct work_struct *work)
> 	vq = vsock->vqs[VSOCK_VQ_TX];
>
> 	for (;;) {
>-		struct scatterlist hdr, buf, *sgs[2];
>+		struct scatterlist *sgs[MAX_SKB_FRAGS + 1];
>+		struct scatterlist bufs[MAX_SKB_FRAGS + 1];

+ 1 is for the header, right?
I'd add a comment just to be clear ;-)

> 		int ret, in_sg = 0, out_sg = 0;
> 		struct sk_buff *skb;
> 		bool reply;
>@@ -111,12 +112,30 @@ 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];
>+
>+		if (skb_is_nonlinear(skb)) {
>+			int i;
>+
>+			for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
>+				struct page *data_page = skb_shinfo(skb)->frags[i].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.
>+				 */

IIUC data_page is a user page, so since we are exposing it to the host,
I think we should pin it.

Is data_page always a user page, or can it be a kernel page when skb is 
nonlinear?

Thanks,
Stefano

>+				sg_init_one(&bufs[i + 1],
>+					    page_to_virt(data_page), PAGE_SIZE);
>+				sgs[out_sg++] = &bufs[i + 1];
>+			}
>+		} else {
>+			if (skb->len > 0) {
>+				sg_init_one(&bufs[1], skb->data, skb->len);
>+				sgs[out_sg++] = &bufs[1];
>+			}
>
>-		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