netdev - Re: [GIT/PATCH v3] xen network backend driver

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Tue, 1 Mar 2011 10:06:49 +0000
From:	Ian Campbell <Ian.Campbell@...citrix.com>
To:	Ben Hutchings <bhutchings@...arflare.com>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	xen-devel <xen-devel@...ts.xensource.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Francois Romieu <romieu@...zoreil.com>
Subject: Re: [GIT/PATCH v3] xen network backend driver

On Mon, 2011-02-28 at 18:53 +0000, Ben Hutchings wrote:
> On Mon, 2011-02-28 at 17:27 +0000, Ian Campbell wrote:

> This should be defined as unsigned long (ideally it would be u64, but
> that can't be updated atomically on 32-bit systems).
[...]
> Don't update last_rx; it's only needed on slave devices of a bond, and
> the bonding driver takes care of it now.

I made these two changes.

> [...]
> > +static int xenvif_change_mtu(struct net_device *dev, int mtu)
> > +{
> > +	struct xenvif *vif = netdev_priv(dev);
> > +	int max = vif->can_sg ? 65535 - ETH_HLEN : ETH_DATA_LEN;
> > +	if (mtu > max)
> > +		return -EINVAL;
> > +	dev->mtu = mtu;
> > +	return 0;
> > +}
> [...]
> 
> Since any VLAN tag must be inserted inline, shouldn't the MTU limit be
> 65535 - VLAN_ETH_HLEN?

In that case shouldn't the other case also be ETH_FRAME_LEN -
VLAN_ETH_HLEN?

I'm not sure what is customary wrt MTU vs VLAN (or other) overheads
under Linux, do we take the hit of the overhead for every device
regardless of VLAN being configured or not or do we expect that people
will configure the MTU as necessary when they configure a VLAN?

Netback itself will cope fine with either MTU, it's the external
connectivity which will actually matter. e.g. the usual configuration
would be (where vifX.Y represents potentially multiple netback devices):

	eth0 <-> eth0.VLAN <-> br0.VLAN <=> vifX.Y

So ultimately it will be the eth0 hardware/driver which matters.

There is a comment in net/8021q/vlan.c which says 
        /* need 4 bytes for extra VLAN header info,
         * hope the underlying device can handle it.
         */
and propagates the underlying device's MTU unmodified so it seems that
the norm is to leave the MTU at maximum assuming no VLAN overhead and
defer any required tweaking to the admin?

Alternatively you might have the VLAN on the eth0 device inside the
guest (e.g. netback<->netfront acts like a trunk link) in which case
basically the same argument applies?

I don't really mind either way so I'm happy to follow whatever the
convention is.

Ian.

--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html