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
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