On 08/19/15 09:42, Yonghyeon PYUN wrote:
On Wed, Aug 19, 2015 at 09:00:52AM +0200, Hans Petter Selasky wrote:
On 08/18/15 23:54, Rick Macklem wrote:
Ouch! Yes, I now see that the code that counts the # of mbufs is before the
code that adds the tcp/ip header mbuf.
In my opinion, this should be fixed by setting if_hw_tsomaxsegcount to
whatever
the driver provides - 1. It is not the driver's responsibility to know if
a tcp/ip
header mbuf will be added and is a lot less confusing that expecting the
driver
author to know to subtract one. (I had mistakenly thought that
tcp_output() had
added the tc/ip header mbuf before the loop that counts mbufs in the list.
Btw,
this tcp/ip header mbuf also has leading space for the MAC layer header.)
Hi Rick,
Your question is good. With the Mellanox hardware we have separate
so-called inline data space for the TCP/IP headers, so if the TCP stack
subtracts something, then we would need to add something to the limit,
because then the scatter gather list is only used for the data part.
I think all drivers in tree don't subtract 1 for
if_hw_tsomaxsegcount. Probably touching Mellanox driver would be
simpler than fixing all other drivers in tree.
Maybe it can be controlled by some kind of flag, if all the three TSO
limits should include the TCP/IP/ethernet headers too. I'm pretty sure
we want both versions.
Hmm, I'm afraid it's already complex. Drivers have to tell almost
the same information to both bus_dma(9) and network stack.
Don't forget that not all drivers in the tree set the TSO limits before
if_attach(), so possibly the subtraction of one TSO fragment needs to go
into ip_output() ....
--HPS
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