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Date:   Tue, 27 Jun 2023 09:49:01 +0200
From:   Stefano Garzarella <sgarzare@redhat.com>
To:     Arseniy Krasnov <avkrasnov@sberdevices.ru>
Cc:     Stefan Hajnoczi <stefanha@redhat.com>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        Bobby Eshleman <bobby.eshleman@bytedance.com>,
        kvm@vger.kernel.org, virtualization@lists.linux-foundation.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel@sberdevices.ru, oxffffaa@gmail.com
Subject: Re: [RFC PATCH v4 03/17] vsock/virtio: support to send non-linear skb
Message-ID: <6g6rfqbfkmfn5or56v25xny6lyhixj6plmrnyg77hirz7dzzhn@jskeqmnbthhk>
References: <20230603204939.1598818-1-AVKrasnov@sberdevices.ru>
 <20230603204939.1598818-4-AVKrasnov@sberdevices.ru>
 <3lg4apldxdrpbkgfa2o4wxe4qyayj2h7b2lfcw3q5a7u3hnofi@z2ifmmzt4xpc>
 <0a89e51b-0f7f-b64b-c827-7c943d6f08a6@sberdevices.ru>
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In-Reply-To: <0a89e51b-0f7f-b64b-c827-7c943d6f08a6@sberdevices.ru>
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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@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?
>
>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
>>>
>>
>