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SubjectRe: [PATCH net-next v7 2/4] vsock/virtio: support to send non-linear skb
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
>

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