linux-kernel - Re: [RFC PATCH v4 03/17] vsock/virtio: support to send non-linear skb

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Message-ID: <6g6rfqbfkmfn5or56v25xny6lyhixj6plmrnyg77hirz7dzzhn@jskeqmnbthhk>
Date:   Tue, 27 Jun 2023 09:49:01 +0200
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Arseniy Krasnov <avkrasnov@...rdevices.ru>
Cc:     Stefan Hajnoczi <stefanha@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Bobby Eshleman <bobby.eshleman@...edance.com>,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel@...rdevices.ru, oxffffaa@...il.com
Subject: Re: [RFC PATCH v4 03/17] vsock/virtio: support to send non-linear skb

On Tue, Jun 27, 2023 at 07:39:41AM +0300, Arseniy Krasnov wrote:
>
>
>On 26.06.2023 18:36, Stefano Garzarella wrote:
>> 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@...rdevices.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?
>
>Yes, non-linear skb always has 'data_len' > 0, e.g. such skbs always have some
>data in it.

Okay, makes sense ;-)

Thanks,
Stefano

>
>Thanks, Arseniy
>
>>
>>> +                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
>>>
>>
>