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From: Thomas Huth <thuth@redhat.com>
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Date: Wed, 10 Feb 2016 11:08:55 +0100
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Subject: Re: [Qemu-devel] [PATCHv7 4/9] slirp: Factorizing tcpiphdr
	structure with an union
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To: Samuel Thibault <samuel.thibault@gnu.org>
Cc: zhanghailiang <zhang.zhanghailiang@huawei.com>, Li Zhijian <lizhijian@cn.fujitsu.com>, Stefan Hajnoczi <stefanha@gmail.com>, Jason Wang <jasowang@redhat.com>, qemu-devel@nongnu.org, Vasiliy Tolstov <v.tolstov@selfip.ru>, Dave Gilbert <dgilbert@redhat.com>, Gonglei <arei.gonglei@huawei.com>, Jan Kiszka <jan.kiszka@siemens.com>, Huangpeng <peter.huangpeng@huawei.com>, Guillaume Subiron <maethor@subiron.org>

On 10.02.2016 10:28, Samuel Thibault wrote:
> That one is tricky, yes :)
> 
> Thomas Huth, on Wed 10 Feb 2016 09:05:32 +0100, wrote:
>>> -#define SLIRP_MSIZE (IF_MTU + IF_MAXLINKHDR + offsetof(struct mbuf, m_dat) + 6)
>>> +#define SLIRP_MSIZE\
>>> +    (offsetof(struct mbuf, m_dat) + IF_MAXLINKHDR + TCPIPHDR_DELTA + IF_MTU)
>>
>> I'm somehow having a hard time to understand why TCPIPHDR_DELTA is used
>> here. As far as I understand, TCPIPHDR_DELTA is the difference between
>> the size of struct tcpiphdr and the size of the IPv4 + TCP header. But
>> if it's just the difference, where does the base size of the headers
>> come from in this define, since the headers are stored in the mbuf, too,
>> aren't they? ... I've got the feeling that I miss something here, could
>> you enlighten me?
> 
> TCP/IP headers are within IF_MTU.

Ah, of course, that makes sense, thanks! ... so in the old definition of
SLIRP_MSIZE, the TCP/IP headers were counted twice, I guess? Anyway, the
definition looks fine to me now.

 Thomas