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Date: Wed, 28 Dec 2022 14:27:35 +0800 From: Jason Wang <jasowang@...hat.com> To: Heng Qi <hengqi@...ux.alibaba.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 2022/12/28 11:50, Heng Qi 写道: > > > 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. Great. Thanks > > 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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