netdev - Re: [ovs-dev] [Xen-devel] Re: [PATCH] ethtool: ETHTOOL_SFEATURES: remove NETIF_F

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Date:	Tue, 31 May 2011 11:43:54 -0700
From:	Jesse Gross <jesse@...ira.com>
To:	Michał Mirosław <mirq-linux@...e.qmqm.pl>
Cc:	Ian Campbell <Ian.Campbell@...rix.com>, dev@...nvswitch.org,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	xen-api@...ts.xensource.com,
	Ben Hutchings <bhutchings@...arflare.com>,
	David Miller <davem@...emloft.net>
Subject: Re: [ovs-dev] [Xen-devel] Re: [PATCH] ethtool: ETHTOOL_SFEATURES:
 remove NETIF_F_COMPAT return

2011/5/29 Michał Mirosław <mirq-linux@...e.qmqm.pl>:
> On Sat, May 28, 2011 at 10:31:03AM -0700, Jesse Gross wrote:
>> 2011/5/28 Ian Campbell <Ian.Campbell@...rix.com>:
>> > On Sat, 2011-05-28 at 08:35 +0100, Michał Mirosław wrote:
>> >> On Sat, May 28, 2011 at 12:25:55AM +0100, Ben Hutchings wrote:
>> >> > On Fri, 2011-05-27 at 18:34 +0200, Michał Mirosław wrote:
>> >> > > On Fri, May 27, 2011 at 04:45:50PM +0100, Ben Hutchings wrote:
>> >> > > > On Fri, 2011-05-27 at 17:28 +0200, Michał Mirosław wrote:
>> >> > [...]
>> >> > > > >      (note: ETHTOOL_S{SG,...} are not ever going away)
>> >> > > > >    - causes NETIF_F_* to be an ABI
>> >> > > > If feature flag numbers are not stable then what is the point of
>> >> > > > /sys/class/net/<name>/features?  Also, I'm not aware that features have
>> >> > > > ever been renumbered in the past.
>> >> > > Since no NETIF_F_* were exported earlier, I assume /sys/class/net/*/features
>> >> > > is a debugging aid. What is it used for besides that?
>> >> > xen-api <https://github.com/xen-org/xen-api> uses it in
>> >> > scripts/InterfaceReconfigureVswitch.py.  Though it doesn't seem to be
>> >> > used for a particularly good reason...
>> >> Look like it should use ETHTOOL_GFLAGS instead for netdev_has_vlan_accel().
>> ETHTOOL_GFLAGS didn't expose the vlan acceleration flags until 2.6.37,
>> which is why /sys/class/net was used instead.
>
> https://github.com/xen-org/xen-api/commit/78b8078e6ae3cf48179859bed6350bb326987546
>
> The commit using it was introduced after 2.6.37 kernel was released

Well people do use kernels other than the most recently released one...

> and uses
> undocumented acccess path to the bits in question. What is the kernel patch
> this commit is referring to?

It's a temporary workaround to deal with the fact that many drivers
that support vlan acceleration do not properly handle vlan packets if
the corresponding group isn't configured on them.
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