netdev - Re: [PATCH 5/7] xen-netfront: reduce gso_max_size to account for ethernet header

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Message-ID: <20130412093433.GD11321@zion.uk.xensource.com>
Date:	Fri, 12 Apr 2013 10:34:34 +0100
From:	Wei Liu <wei.liu2@...rix.com>
To:	Ian Campbell <Ian.Campbell@...rix.com>
CC:	Wei Liu <wei.liu2@...rix.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	David Vrabel <david.vrabel@...rix.com>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
	"annie.li@...cle.com" <annie.li@...cle.com>,
	"wdauchy@...il.com" <wdauchy@...il.com>
Subject: Re: [PATCH 5/7] xen-netfront: reduce gso_max_size to account for
 ethernet header

On Fri, Apr 12, 2013 at 09:57:15AM +0100, Ian Campbell wrote:
> On Fri, 2013-04-12 at 09:48 +0100, Wei Liu wrote:
> > On Fri, Apr 12, 2013 at 09:18:04AM +0100, Ian Campbell wrote:
> > > On Thu, 2013-04-11 at 21:04 +0100, Wei Liu wrote:
> > > > On Tue, Apr 09, 2013 at 12:07:33PM +0100, Wei Liu wrote:
> > > > > The maximum packet including ethernet header that can be handled by netfront /
> > > > > netback wire format is 65535. Reduce gso_max_size accordingly.
> > > > > 
> > > > > Drop skb and print warning when skb->len > 65535. This can 1) save the effort
> > > > > to send malformed packet to netback, 2) help spotting misconfiguration of
> > > > > netfront in the future.
> > > > > 
> > > > 
> > > > Any opinion on how much space should be reserved? From a previous thread
> > > > Ben seemed to suggest 90 (Ethernet + VLAN tag + IPv6 + TCP + timestamp
> > > > option = 90 bytes).
> > > 
> > > I trust Ben and that seems as good as anything to me.
> > > 
> > > Is this the sort of limit others might be interested in, should we have
> > > a global #define?
> > > 
> > 
> > We shall have a global define in this case.
> > 
> > #define XEN_NETFRONT_MAX_HEADER ? I'm bad at naming things.
> 
> I meant an include/linux/skbuff.h (or some suitable header) #define
> SKB_MAX_FOO type thing...
> 

But we don't have handle on this. If I understand correctly the
discussion in other thread, 90 is empirical value, not something
documented.


Wei.

> Ian.
> 
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