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Re: [Xen-devel] Re: [PATCH] Remus breaks the build

To: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Re: [PATCH] Remus breaks the build
From: Brendan Cully <brendan@xxxxxxxxx>
Date: Wed, 18 Aug 2010 10:58:52 -0700
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, "Xen-devel@xxxxxxxxxxxxxxxxxxx" <Xen-devel@xxxxxxxxxxxxxxxxxxx>, Ian Campbell <Ian.Campbell@xxxxxxxxxx>, Stabellini <stefano.stabellini@xxxxxxxxxxxxx>, Stefano@xxxxxxxxx, Dulloor <dulloor@xxxxxxxxx>
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On Wednesday, 18 August 2010 at 14:40, Ian Jackson wrote:
> Brendan Cully writes ("Re: [Xen-devel] Re: [PATCH] Remus breaks the build"):
> > We need a second module (IFB or IMQ, depending on the kernel version)
> > because Linux queueing disciplines only operate on a device's outbound
> > traffic. Since Remus runs in dom0, it sees the guest's outbound
> > traffic as _inbound_ traffic on a VIF device. So IMQ/IFB is used to
> > redirect that incoming VIF traffic to a virtual intermediate device
> > with the sch_queue queueing discipline installed on it.
> Couldn't this be achieved simply by putting a dummy bridge or tap
> device in the way or something ?  You seem to be describing a device
> driver whose sole purpose is plumbing ...

You are correct, the device is just plumbing. But it isn't any extra
code, at least for pvops -- IFB is carried in upstream linux. You
could probably use a second-level bridge or tap or policy routing or
something to get the same effect, but I don't think I see the
advantage. You're certainly welcome to put together an alternative
approach for comparison.

What would be simpler and more efficient would be to move the queueing
directly into netback. That's on the todo list but hasn't gotten
attention yet.

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