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Re: [Xen-devel] [patch xen.git xen-tip/master] xen: fix xenbus frontend

To: M A Young <m.a.young@xxxxxxxxxxxx>
Subject: Re: [Xen-devel] [patch xen.git xen-tip/master] xen: fix xenbus frontend build
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Wed, 06 May 2009 15:48:52 -0700
Cc: Randy Dunlap <randy.dunlap@xxxxxxxxxx>, virtualization@xxxxxxxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxx, Jeremy Fitzhardinge <jeremy@xxxxxxxxxxxxx>, Chris Wright <chrisw@xxxxxxxxxxxx>
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M A Young wrote:
On Tue, 5 May 2009, Randy Dunlap wrote:

From: Randy Dunlap <randy.dunlap@xxxxxxxxxx>

When a driver kconfig symbol =m and it selects another symbol,
that other symbol will also be =m (unless something else
causes it to be =y), so when XEN_BLKDEV_FRONTEND=m and/or
XEN_NETDEV_FRONTEND=m, then XEN_XENBUS_FRONTEND=m, but that
won't build (build error message below).  Changing
XEN_XENBUS_FRONTEND from a tristate to a bool makes it be
=y (builtin) any time that it is selected, so there is
no build error.

That isn't the right solution. The real problem is that something you have selected as "y" does depend on XEN_XENBUS_FRONTEND but doesn't select it. Switching XEN_XENBUS_FRONTEND from tristate to bool might fix your particular compile problem, but it means that the situation you would get if you changed your configuration so that XEN_BLKDEV_FRONTEND=n and XEN_NETDEV_FRONTEND=n (likewise any other options that do select XEN_XENBUS_FRONTEND) would still broken because then XEN_XENBUS_FRONTEND won't be selected at all.

If your configuration has XEN_PCI_PASSTHROUGH=y then I posted a patch for this very situation a few days ago (and it is now in xen-tip/next, though wasn't yet in xen-tip/master when I last checked).

pcifront wasn't meant to go into master yet. I'd be interested in some testing feedback from next.

These xenbus config options seem to be in a bit of a mess, and it would be nice if someone could go through and make them sane. I think the ideal outcome should be:

   * the backend and frontend should be independently modular
   * they should be independently selectable (in principle we should
     allow a backend-only kernel, which I don't think is possible atm)
   * rather than having separate configs for frontends and backends,
     and a config frontend/backend xenbus, why not make the drivers
     depend on their appropriate xenbus, and directly configure them?

Anyone?
   J

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