Re: [RFC PATCH v1 05/12] vsock/virtio: non-linear skb support

[Krasnov Arseniy <AVKrasnov@sberdevices.ru>]

On 16.02.2023 17:18, Stefano Garzarella wrote:
> 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?

By default it is user page, but...may be it is possible to send page of mapped
file with MAP_SHARED flags(i think it this case it will be page from page cache)

> 
> 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
>