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SubjectRe: [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@sberdevices.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

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