[Xen-devel] Question on sg in netback tx path

["James Harper" <james.harper@xxxxxxxxxxxxxxxx>]

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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