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

[Andre Oppermann]

On 05.06.2013 08:13, 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).

We do support 4K mbufs and have done so for a long time.  The problem is
that socket buffer mbuf chains can be any combination of mbuf sizes and
m_defrag() so far only collapses to 2K mbuf clusters.  The latter can be
changed but it is used in a number of places where an explicit 2K assumption
may have been made (even if it shouldn't).  When all them are checked
m_defrag() can be changed to collapse into 4K mbufs and this "hack" removed.

-- 
Andre