Xen devel - Mar 2008 - Question on sg in netback tx path

I''m trying to improve performance on the gpl pv drivers for windows,
and
am running into trouble with the sg stuff on the tx path.

Taking an icmp packet as an example, windows formats the packet like
this:
sg entry 0 - Ethernet header
sg entry 1 - IP header
sg entry 2 - payload

Those pieces could be anywhere inside the page.

In netback.c, it appears that the sg magic happens inside
netbk_count_requests. The following statement confuses me:

"
                if (txp->size > first->size) {
                        DPRINTK("Frags galore\n");
                        return -frags;
                }
"

If I interpret the implications of that correctly, the ''size''
field in
the first fragment of a packet should be the size of the entire packet,
but then how do I tell netback how long my first fragment is?

Thanks

James


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> -----Original Message-----
> From: xen-devel-bounces@lists.xensource.com
> [mailto:xen-devel-bounces@lists.xensource.com] On Behalf Of
> James Harper
> Sent: Monday, March 03, 2008 9:03 PM
> To: xen-devel@lists.xensource.com
> Subject: [Xen-devel] Question on sg in netback tx path
>
> I''m trying to improve performance on the gpl pv drivers for
> windows, and am running into trouble with the sg stuff on the tx path.
>
> Taking an icmp packet as an example, windows formats the packet like
> this:
> sg entry 0 - Ethernet header
> sg entry 1 - IP header
> sg entry 2 - payload
>
> Those pieces could be anywhere inside the page.
>
> In netback.c, it appears that the sg magic happens inside
> netbk_count_requests. The following statement confuses me:
>
> "
>                 if (txp->size > first->size) {
>                         DPRINTK("Frags galore\n");
>                         return -frags;
>                 }
> "
>
> If I interpret the implications of that correctly, the
''size''
> field in the first fragment of a packet should be the size of
> the entire packet,
  Correct.
>but then how do I tell netback how long my
> first fragment is?
  Look at the line following the code above:
                first->size -= txp->size;

  This line subtract each fragment size from the "size" field in the
first fragment.
  When the loop ends you will have subtracted the size of all fragments and
first->size will now have the actual size of the first fragment.

Renato
>
> Thanks
>
> James
>
>
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>
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> > If I interpret the implications of that correctly, the
''size''
> > field in the first fragment of a packet should be the size of
> > the entire packet,
> 
>   Correct.
> 
> >but then how do I tell netback how long my
> > first fragment is?
> 
>   Look at the line following the code above:
>                 first->size -= txp->size;
> 
>   This line subtract each fragment size from the "size" field in
the
first
> fragment.
>   When the loop ends you will have subtracted the size of all
fragments
> and first->size will now have the actual size of the first fragment.
> 
Thanks for the clarification - all makes sense now!

James

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