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: <m21pnw91js.wl-thehajime@gmail.com>
Date: Wed, 24 Sep 2025 08:44:39 +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 Mon, 22 Sep 2025 15:32:22 +0900,
Johannes Berg wrote:
> 
> On Sat, 2025-09-20 at 08:46 +0900, Hajime Tazaki wrote:
> > diff --git a/arch/um/drivers/Kconfig b/arch/um/drivers/Kconfig
> > index 6a0354ca032f..04025207a077 100644
> > --- a/arch/um/drivers/Kconfig
> > +++ b/arch/um/drivers/Kconfig
> > @@ -159,6 +159,7 @@ config UML_RTC
> >  
> >  config UML_PCI
> >         bool
> > +       depends on MMU
> 
> That won't do anything since you elsewhere have "select UML_PCI"
> independent of MMU.

i see.

> > @@ -170,6 +171,7 @@ config UML_PCI_OVER_VIRTIO
> >         bool "Enable PCI over VIRTIO device simulation"
> >         # in theory, just VIRTIO is enough, but that causes recursion
> >         depends on VIRTIO_UML
> > +       depends on MMU
> >         select UML_PCI
> 
> Right, but you also need that for UML_PCI_OVER_VFIO.

thanks, I understand too.

> > and do
> >   ./tools/testing/kunit/kunit.py config  --kconfig_add CONFIG_MMU=n
> > 
> > the validation currently gives the following error:
> > 
> >  ERROR:root:Not all Kconfig options selected in kunitconfig were in the generated .config.
> >  This is probably due to unsatisfied dependencies.
> >  Missing: CONFIG_UML_PCI_OVER_VIRTIO=y
> 
> Well, OK, but that's fair - you did specifically override MMU=n, and
> virtio-over-pci needs it.
> 
> > 1) use --kconfig_add CONFIG_UML_PCI_OVER_VIRTIO=n when using kunit w/
> >   !MMU, and drop this patch from the series (no modification to the tree)
> > 2) prepare a different file for !MMU & ARCH=um testing (e.g.,
> >   arch_uml_nommu.config), and add an option to kunit.py to switch MMU
> >   or !MMU
> > 3) implement virtio-pci for !MMU and propose to remove the restriction
> >   of CONFIG_PCI depends on CONFIG_MMU.
> > 
> > 2) will be removed when 3) is done so, I'm hesitating to propose a
> > patch used by whole tree.
> > 
> > so, I think 1) is (not the best but) a reasonable solution, with a
> > note in nommu-uml specific document (i.e., [PATCH 12/13]).
> 
> I don't think (3) makes any sense at all, we should just _never_ do
> that. !MMU is really here in UML for testing to support other
> architectures that are !MMU, and since by today's definitions no other
> architecture can have PCI without MMU, it makes no sense for UML to have
> that (and complicate the PCI code unnecessarily, etc.)
> 
> I think it's entirely reasonable to have overriding CONFIG_MMU=n to also
> necessitate overriding CONFIG_UML_PCI_OVER_VIRTIO, i.e. (1).

okay, I'll go for this direction in the next series.

> As to whether or not to add a specific config file, honestly I don't
> really know or even care - you'd have to ask the people who actually
> want to test !MMU.

indeed, this will be the out of this series if needed.

-- Hajime