Re: arch/xen is a bad idea

From: Andi Kleen
Date: Fri Dec 17 2004 - 01:06:04 EST

On Thu, Dec 16, 2004 at 09:00:52PM +0000, Ian Pratt wrote:
> > I guess if we were to go the way which Ian is proposing it would be
> >
> > a) Add arch/xen
> >
> > b) Spend N weeks integrating xen into arch/i386, while also separately
> > maintaining arch/xen.
> >
> > c) Remove arch/xen
> >
> > So... why not skip a), c) and half of b)?
> That's not quite what I'm proposing.
> Once arch/xen is in the tree, we'd start submitting patches that
> try and unify more of arch xen and i386 to reduce the number of
> files that we have to modify.

IMHO reducing the number of files to modify should be done
first before attempting a mainline merge. That is how a lot
of other more intrusive changes have been merged in the past.

There was a intensive review process and eventually the interfaces
were cleaned up a lot of and the thing was merged piece by piece
after considerable changes.

> I think we'd then have to make a decision as to whether merging
> i386 and xen/i386 is feasible. Further, we'd have to make a
> decision as to whether what is really wanted is a single kernel
> that's able to boot-time switch between native i386 and xen,
> rather than just having a single source base. The two options
> would probably result in rather different implementations.

Even without boot time switch but only CONFIG switch
a more shared code base would be probably the only saner
way long time. I'm not sure a run time switch is really
that good an idea, it would essentially require to run
most of pgtable.h etc. through function pointers and
I don't like this idea very much. But at least doing the
reorganization on the source code level would probably
clean up the interface considerably.

> In short, merging is far from trivial, and its not even clear
> quite what is wanted.
> I'm not convinced that maintaining xen/i386 in its current form
> is going to be as hard as Andi thinks. We already share many
> files unmodified from i386. Keeping i386 and xen/i386 in sync is
> fairly mechanical: we can apply most of the patches to i386 to
> xen/i386 directly.

Perhaps for now, but eventually this will not work anymore due
to inevitable code drift (I went exactly through the same phase for a year
or so on x86-64. These days I have to redo most patches unless
i'm lucky enough that the contributor does both)

And with also supporting x86-64 it will not really work this
way well because you would need to merge from two sources.
I can't see any mechanical scheme handling that.
The only sane way to maintain the merging scheme you proposed
would be then arch/xen64, multiplying work again.

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