linux-kernel - Re: [PATCH RESEND v11 10/13] um: nommu: a work around for MMU dependency to PCI driver

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Message-ID: <m27bxu949d.wl-thehajime@gmail.com>
Date: Fri, 19 Sep 2025 18:32:30 +0900
From: Hajime Tazaki <thehajime@...il.com>
To: johannes@...solutions.net
Cc: linux-um@...ts.infradead.org,
	ricarkol@...gle.com,
	Liam.Howlett@...cle.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND v11 10/13] um: nommu: a work around for MMU dependency to PCI driver

On Fri, 19 Sep 2025 16:24:07 +0900,
Johannes Berg wrote:
> 
> On Fri, 2025-09-19 at 09:03 +0900, Hajime Tazaki wrote:
> > > This doesn't make a lot of sense to me. Why would we even want to build
> > > PCI on NOMMU-UML if PCI in general is dependent on MMU now?
> > > 
> > > It's not like ARCH=um with PCI and NOMMU has any value even for testing
> > > if such a configuration cannot exist in reality?
> > 
> > totally understand your point.
> > 
> > now I see that we don't have to have this work around by using
> > --kconfig_add option to kunit.py.
> > 
> > # like --kconfig_add CONFIG_UML_PCI_OVER_VIRTIO=n (in addition to
> >   --kconfig_add CONFIG_MMU=n).
> 
> That's not what I mean. I think it should be made impossible to build
> the broken code.

okay.
# I think now I lost the point...

currently, drivers/pci/Kconfig (CONFIG_PCI) marks as depends on MMU,
so we cannot select it when CONFIG_MMU=n.

but it's different with kunit when using them via kunit.py config,

it first adds

 CONFIG_VIRTIO_UML=y
 CONFIG_UML_PCI_OVER_VIRTIO=y

via tools/testing/kunit/configs/arch_uml.config, and then add

 CONIFG_MMU=n

via --kconfig_add CONFIG_MMU=n.

and then execute make ARCH=um olddefconfig, which in turn enables
CONFIG_UML_PCI_OVER_VIRTIO.

if we append "--kconfig_add CONFIG_UML_PCI_OVER_VIRTIO=n" to kunit.py,
it will overwrite the arch_uml.config.

# I don't know how kunit handles those appended CONFIG entries, though..

my goal is simple; to test !MMU code via kunit.
my original patch or the additional kconfig argument (--kconfig_add)
satisfies this goal.

> The problem is probably UML_PCI_OVER_VIRTIO selecting UML_PCI selecting
> various PCI code, but nothing depends on PCI in the first place. Which
> it should, then?

I don't understand the 'nothing depends on PCI...' part.  care to
elaborate ?

thanks,

-- Hajime