svn commit: r251297 - head/sys/dev/xen/netfront

[Kenneth D. Merry]

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
ken at FreeBSD.ORG