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RE: [PATCH for-7.2] rtl8139: honor large send MSS value

From: Tobias Fiebig
Subject: RE: [PATCH for-7.2] rtl8139: honor large send MSS value
Date: Thu, 17 Nov 2022 12:26:57 +0100

Ok, that explains a lot. I was also thinking that the vlan bit seem to overlap 
with the MTU field, and wanted to look at that later today.

Re the 12b: IIRC, the standard 1500 MTU for ethernet is already without the 
ethernet header; That can have up to 26b (18b basis, 4b 802.1q, 4b 802.1ad), 
but leads to a total frame length of 1526 (with other additions (MPLS) also 
just making the frame bigger, without touching the MTU/MSS). MSS than as usual 
-40 for v4 and ~ -60 for v6.

So I doubt that those 12b are subtracted for the ethernet header.

Does somebody still have an RTL8139 around, to test how the real hardware 

With best regards,

-----Original Message-----
From: Stefan Hajnoczi <stefanha@gmail.com> 
Sent: Thursday, 17 November 2022 12:16
To: Tobias Fiebig <tobias@fiebig.nl>
Cc: Jason Wang <jasowang@redhat.com>; Stefan Hajnoczi <stefanha@redhat.com>; 
qemu-devel@nongnu.org; qemu-stable@nongnu.org; Russell King - ARM Linux 
Subject: Re: [PATCH for-7.2] rtl8139: honor large send MSS value

After looking more closely at txdw0 it seems that the code mixes "Tx command 
mode 0", "Tx command mode 1", and "Tx status mode". The bits have different 
meanings in each mode, so this leads to confusion :).


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