netdev - Re: [PATCH] net: xen-netback: convert to hw

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Message-ID: <20110419134352.GB4716@rere.qmqm.pl>
Date:	Tue, 19 Apr 2011 15:43:52 +0200
From:	Michał Mirosław <mirq-linux@...e.qmqm.pl>
To:	Ian Campbell <Ian.Campbell@...citrix.com>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>
Subject: Re: [PATCH] net: xen-netback: convert to hw_features

On Tue, Apr 19, 2011 at 02:39:00PM +0100, Ian Campbell wrote:
> On Tue, 2011-04-19 at 14:30 +0100, Michał Mirosław wrote:
> > On Tue, Apr 19, 2011 at 02:17:53PM +0100, Ian Campbell wrote:
> > > I fixed it with the following, I also moved the !can_sg MTU clamping
> > > into a set_features hook (like we do with netfront). Am I right that
> > > this pattern copes with changes to SG via ethtool etc better? I think
> > > it's more future proof in any case.
> > This looks wrong. Even if SG is turned on, you might get big skbs which
> > are linearized. There is a difference in SG capability and SG offload
> > status and as I see it the capability is what you need to test for MTU.
> So the existing stuff in drivers/net/xen-netfront.c is wrong too?

Looks like it. But I don't really know what are the real constraints for MTU.
What I know is that SG even if turned on needs not be used (and currently
it's not e.g. if checksum offload is disabled). So MTU setting should not
depend on SG offload state but on some capability.

Best Regards,
Michał Mirosław
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