Of all the gin joints in all the towns in all the world, Kenneth D.
Merry had to walk into mine and say:
> > Talking of the XMAC II, there's one other thing I forgot to mention earlier.
> > The FreeBSD sk driver does jumbo frames, but the SysKonnect drivers don't.
> > At least, not yet. The XMAC II's receive FIFO is 8K. By default, the chip
> > operates in 'store and forward' mode in order to perform error checking on
> > received frames (it has to get the entire frame in the FIFO in order to
> > do a CRC on it, I think). This is fine for normal frames, but if you want
> > to handle jumbograms larger than 8192 bytes, you have to put the chip into
> > 'streaming' mode, otherwise any frame larger than 8192 bytes will be truncated.
> > To get 'streaming' mode to work, you have to disable all of the RX error
> > checking.
>
> That is unfortunate, since it means you can't do checksum offloading with
> jumbo frames.
Uhm. I'm not sure about that. The 8K FIFO limitation is in the XMAC II,
not in the GEnesis controller. And I believe it's the GEnesis that actually
does the hardware checksumming stuff.
Oh, and the XMAC appears to have a 4K TX FIFO, not 2K. My mistake.
> FWIW, of the three gigabit ethernet implementations I've seen anything of
> (Alteon, Intel, SysKonnect), none have implemented all of the hooks
> necessary for a seamless zero copy receive implementation.
>
> Alteon comes the closest, but they don't support splitting out the headers
> (yet), which is a requirement for us. The only way to do zero copy receive
> with our VM architecture (that I know of) is page flipping, i.e. receive
> the page in the kernel, and then trade it for the user's page. You can't
> do it on anything less than page-sized granularity, and things have to be
> page aligned. (The IO-Lite stuff from Rice is an exception to all this.)
>
> The nice thing about the Alteon boards, though, is that you can modify the
> firmware, and so header splitting is an option there. It would even be
> possible to split the headers off of IPv6 packets, or any other protocol
> that you have knowlege of.
If you can actually modify the firmware to do this then you have a lot
more guru points than I do. :) I've looked at the Alteon firmware code
but it's all quite opaque to me.
-Bill
--
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-Bill Paul (212) 854-6020 | System Manager, Master of Unix-Fu
Work: [EMAIL PROTECTED] | Center for Telecommunications Research
Home: [EMAIL PROTECTED] | Columbia University, New York City
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"It is not I who am crazy; it is I who am mad!" - Ren Hoek, "Space Madness"
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