On Tue, Jun 04, 2013 at 23:13:54 -0700, Colin Percival wrote: > On 06/04/13 22:51, Lawrence Stewart wrote: > > On 06/03/13 23:00, Andre Oppermann wrote: > >> Modified: head/sys/dev/xen/netfront/netfront.c > >> ============================================================================== > >> --- head/sys/dev/xen/netfront/netfront.c Mon Jun 3 12:55:13 2013 > >> (r251296) > >> +++ head/sys/dev/xen/netfront/netfront.c Mon Jun 3 13:00:33 2013 > >> (r251297) > >> @@ -134,6 +134,7 @@ static const int MODPARM_rx_flip = 0; > >> * to mirror the Linux MAX_SKB_FRAGS constant. > >> */ > >> #define MAX_TX_REQ_FRAGS (65536 / PAGE_SIZE + 2) > >> +#define NF_TSO_MAXBURST ((IP_MAXPACKET / PAGE_SIZE) * MCLBYTES) > > > > For posterity's sake, can you and/or Colin please elaborate on how this > > value was determined and what it is dependent upon? Could a newer > > version of Xen remove the need for this reduced limit? > > The comment above (of which only the last line is quoted in the diff) > explains it: > * This limit is imposed by the backend driver. We assume here that > * we are dealing with a Linux driver domain and have set our limit > * to mirror the Linux MAX_SKB_FRAGS constant. > > This isn't a Xen issue really; rather, it's a Linux Dom0 issue. AFAIK > there are no changes in the pipe to fix this in Linux; but this would not > be needed with a different Dom0 (e.g., a FreeBSD Dom0, if/when that becomes > possible) or if FreeBSD switched to using 4kB mbuf clusters (since at that > point we would be matching Linux and be able to fit a maximum-length IP > packet into the allowed number of fragments).
It has been a couple of years since I looked at that, but IIRC one of the issues here is that the is no way to probe for the maximum. So even once we have a FreeBSD Dom 0, we may be stuck with the limit. Or we'll have to have some other way (e.g. the xenstore) to communicate that we're running on a different type of Dom 0 and don't have the limit. Ken -- Kenneth Merry k...@freebsd.org _______________________________________________ svn-src-head@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
