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Date: Wed, 28 Dec 2022 11:50:52 +0800 From: Heng Qi <hengqi@...ux.alibaba.com> To: Jason Wang <jasowang@...hat.com>, netdev@...r.kernel.org, bpf@...r.kernel.org Cc: "Michael S . Tsirkin" <mst@...hat.com>, Paolo Abeni <pabeni@...hat.com>, Jakub Kicinski <kuba@...nel.org>, John Fastabend <john.fastabend@...il.com>, "David S . Miller" <davem@...emloft.net>, Daniel Borkmann <daniel@...earbox.net>, Alexei Starovoitov <ast@...nel.org>, Eric Dumazet <edumazet@...gle.com>, Xuan Zhuo <xuanzhuo@...ux.alibaba.com> Subject: Re: [PATCH v2 2/9] virtio_net: set up xdp for multi buffer packets 在 2022/12/27 下午8:20, Heng Qi 写道: > > > 在 2022/12/27 下午2:32, Jason Wang 写道: >> >> 在 2022/12/20 22:14, Heng Qi 写道: >>> When the xdp program sets xdp.frags, which means it can process >>> multi-buffer packets over larger MTU, so we continue to support xdp. >>> But for single-buffer xdp, we should keep checking for MTU. >>> >>> Signed-off-by: Heng Qi <hengqi@...ux.alibaba.com> >>> Reviewed-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com> >>> --- >>> drivers/net/virtio_net.c | 4 ++-- >>> 1 file changed, 2 insertions(+), 2 deletions(-) >>> >>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c >>> index 443aa7b8f0ad..c5c4e9db4ed3 100644 >>> --- a/drivers/net/virtio_net.c >>> +++ b/drivers/net/virtio_net.c >>> @@ -3095,8 +3095,8 @@ static int virtnet_xdp_set(struct net_device >>> *dev, struct bpf_prog *prog, >>> return -EINVAL; >>> } >>> - if (dev->mtu > max_sz) { >>> - NL_SET_ERR_MSG_MOD(extack, "MTU too large to enable XDP"); >>> + if (prog && !prog->aux->xdp_has_frags && dev->mtu > max_sz) { >> >> >> Not related to this patch, but I see: >> >> unsigned long int max_sz = PAGE_SIZE - sizeof(struct >> padded_vnet_hdr); >> >> Which is suspicious, do we need to count reserved headroom/tailroom >> as well? > > This seems to be suspicious. After loading xdp, the size of the filled > avail buffer > is (PAGE_SIZE - headroom - tailroom), so the size of the received used > buffer, ie MTU, > should also be (PAGE_SIZE - headroom - tailroom). Hi Jason, this is indeed a problem. After verification, packet drop will indeed occur. To avoid this, the size of MTU should be (PAGE_SIZE - headroom - tailroom - ethhdr = 4096 - 256 -320 - 14 =3506). Because when there is xdp, each filling is 3520 (PAGE_SIZE - room), if the value of (MTU + 14) is greater than 3520 (because the MTU does not contain the ethernet header), then the packet with a length greater than 3520 will come in, so num_buf will still be greater than or equal to 2, and then xdp_linearize_page() will be performed and the packet will be dropped because the total length is greater than PAGE_SIZE. I will make a separate bugfix patch to fix this later. Thanks. > > Thanks. > >> >> Thanks >> >> >>> + NL_SET_ERR_MSG_MOD(extack, "MTU too large to enable XDP >>> without frags"); >>> netdev_warn(dev, "XDP requires MTU less than %lu\n", max_sz); >>> return -EINVAL; >>> }
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