Date: Tue, 04 Jun 2013 23:13:54 -0700 From: Colin Percival <cperciva@freebsd.org> To: Lawrence Stewart <lstewart@freebsd.org> Cc: svn-src-head@FreeBSD.org, svn-src-all@FreeBSD.org, src-committers@FreeBSD.org, Andre Oppermann <andre@FreeBSD.org> Subject: Re: svn commit: r251297 - head/sys/dev/xen/netfront Message-ID: <51AED722.8000504@freebsd.org> In-Reply-To: <51AED1F3.2060003@freebsd.org> References: <201306031300.r53D0XUx092178@svn.freebsd.org> <51AED1F3.2060003@freebsd.org>
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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). -- Colin Percival Security Officer Emeritus, FreeBSD | The power to serve Founder, Tarsnap | www.tarsnap.com | Online backups for the truly paranoid
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