Re: [PATCH net-next 21/33] virtio_net: add XDP frame size in two code paths

[Jason Wang <jasowang@redhat.com>]

On 2020/4/28 下午5:50, Jason Wang wrote:
>
> On 2020/4/27 下午10:32, Jesper Dangaard Brouer wrote:
>> On Mon, 27 Apr 2020 15:21:02 +0800
>> Jason Wang<jasowang@redhat.com> wrote:
>>
>>> On 2020/4/23 上午12:09, Jesper Dangaard Brouer wrote:
>>>> The virtio_net driver is running inside the guest-OS. There are two
>>>> XDP receive code-paths in virtio_net, namely receive_small() and
>>>> receive_mergeable(). The receive_big() function does not support XDP.
>>>>
>>>> In receive_small() the frame size is available in buflen. The buffer
>>>> backing these frames are allocated in add_recvbuf_small() with same
>>>> size, except for the headroom, but tailroom have reserved room for
>>>> skb_shared_info. The headroom is encoded in ctx pointer as a value.
>>>>
>>>> In receive_mergeable() the frame size is more dynamic. There are two
>>>> basic cases: (1) buffer size is based on a exponentially weighted
>>>> moving average (see DECLARE_EWMA) of packet length. Or (2) in case
>>>> virtnet_get_headroom() have any headroom then buffer size is
>>>> PAGE_SIZE. The ctx pointer is this time used for encoding two values;
>>>> the buffer len "truesize" and headroom. In case (1) if the rx buffer
>>>> size is underestimated, the packet will have been split over more
>>>> buffers (num_buf info in virtio_net_hdr_mrg_rxbuf placed in top of
>>>> buffer area). If that happens the XDP path does a xdp_linearize_page
>>>> operation.
>>>>
>>>> Cc: Jason Wang<jasowang@redhat.com>
>>>> Signed-off-by: Jesper Dangaard Brouer<brouer@redhat.com>
>>>> ---
>>>>    drivers/net/virtio_net.c |   15 ++++++++++++---
>>>>    1 file changed, 12 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>>>> index 11f722460513..1df3676da185 100644
>>>> --- a/drivers/net/virtio_net.c
>>>> +++ b/drivers/net/virtio_net.c
>>>> @@ -689,6 +689,7 @@ static struct sk_buff *receive_small(struct 
>>>> net_device *dev,
>>>>            xdp.data_end = xdp.data + len;
>>>>            xdp.data_meta = xdp.data;
>>>>            xdp.rxq = &rq->xdp_rxq;
>>>> +        xdp.frame_sz = buflen;
>>>>            orig_data = xdp.data;
>>>>            act = bpf_prog_run_xdp(xdp_prog, &xdp);
>>>>            stats->xdp_packets++;
>>>> @@ -797,10 +798,11 @@ static struct sk_buff 
>>>> *receive_mergeable(struct net_device *dev,
>>>>        int offset = buf - page_address(page);
>>>>        struct sk_buff *head_skb, *curr_skb;
>>>>        struct bpf_prog *xdp_prog;
>>>> -    unsigned int truesize;
>>>> +    unsigned int truesize = mergeable_ctx_to_truesize(ctx);
>>>>        unsigned int headroom = mergeable_ctx_to_headroom(ctx);
>>>> -    int err;
>>>>        unsigned int metasize = 0;
>>>> +    unsigned int frame_sz;
>>>> +    int err;
>>>>           head_skb = NULL;
>>>>        stats->bytes += len - vi->hdr_len;
>>>> @@ -821,6 +823,11 @@ static struct sk_buff 
>>>> *receive_mergeable(struct net_device *dev,
>>>>            if (unlikely(hdr->hdr.gso_type))
>>>>                goto err_xdp;
>>>>    +        /* Buffers with headroom use PAGE_SIZE as alloc size,
>>>> +         * see add_recvbuf_mergeable() + get_mergeable_buf_len()
>>>> +         */
>>>> +        frame_sz = headroom ? PAGE_SIZE : truesize;
>>>> +
>>>>            /* This happens when rx buffer size is underestimated
>>>>             * or headroom is not enough because of the buffer
>>>>             * was refilled before XDP is set. This should only
>>>> @@ -834,6 +841,8 @@ static struct sk_buff *receive_mergeable(struct 
>>>> net_device *dev,
>>>>                                  page, offset,
>>>>                                  VIRTIO_XDP_HEADROOM,
>>>>                                  &len);
>>>> +            frame_sz = PAGE_SIZE;
>>> Should this be PAGE_SIZE -  SKB_DATA_ALIGN(sizeof(struct 
>>> skb_shared_info))?
>> No, frame_sz include the SKB_DATA_ALIGN(sizeof(struct 
>> skb_shared_info)) length.
>
>
> Ok, consider mergeable buffer path depends on the truesize which is 
> encoded in ctx.
>
> It looks to the the calculation in add_recvfbuf_mergeable() is wrong, 
> we need count both headroom and tailroom there.
>
> We probably need the attached 2 patches to fix this.
>
> (untested, will test it tomorrow).


Sorry for the late reply. I gave a test and post the attached two 
patches (with minor tweaks).

It looks to me they are required for this patch to work since 
data_hard_start excludes vnet hdr len without the attached patches which 
means PAGE_SIZE could not be used as frame_sz.

Thanks


>
> Thanks