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    SubjectRe: [RFC PATCH v4 03/17] vsock/virtio: support to send non-linear skb
    On Sat, Jun 03, 2023 at 11:49:25PM +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>
    >---
    > 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...

    >+

    For readability, I would move the smaller branch above:

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

    >+ 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?

    >+ sg_init_one(&bufs[1], skb->data, skb->len);
    >+ sgs[out_sg++] = &bufs[1];
    >+ }
    >
    ^
    Blank line that we can remove.

    Stefano

    >- 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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