linux-kernel - Re: [Xen-devel] [PATCH v2 2/2] x86, arm, platform, xen, kconfig: add xen defconfig helper

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Date:	Wed, 14 Jan 2015 19:36:36 +0100
From:	"Luis R. Rodriguez" <mcgrof@...e.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
Cc:	"Luis R. Rodriguez" <mcgrof@...not-panic.com>, hpa@...or.com,
	josh@...htriplett.org, sam@...nborg.org,
	Michal Marek <mmarek@...e.cz>,
	Randy Dunlap <rdunlap@...radead.org>,
	Ian Campbell <Ian.Campbell@...rix.com>, kvm@...r.kernel.org,
	x86@...nel.org, linux-kernel@...r.kernel.org, mtosatti@...hat.com,
	Pekka Enberg <penberg@...nel.org>, fengguang.wu@...el.com,
	levinsasha928@...il.com, David Rientjes <rientjes@...gle.com>,
	xen-devel@...ts.xenproject.org, Borislav Petkov <bp@...e.de>,
	David Vrabel <david.vrabel@...rix.com>, bpoirier@...e.de
Subject: Re: [Xen-devel] [PATCH v2 2/2] x86, arm, platform, xen, kconfig:
	add xen defconfig helper

On Wed, Jan 14, 2015 at 11:29:41AM +0000, Stefano Stabellini wrote:
> On Tue, 13 Jan 2015, Luis R. Rodriguez wrote:
> > On Mon, Dec 15, 2014 at 02:58:26PM +0000, Stefano Stabellini wrote:
> > > On Tue, 9 Dec 2014, Luis R. Rodriguez wrote:
> > > > From: "Luis R. Rodriguez" <mcgrof@...e.com>
> > > > 
> > > > This lets you build a kernel which can support xen dom0
> > > > or xen guests by just using:
> > > > 
> > > >    make xenconfig
> > > > 
> > > > on both x86 and arm64 kernels. This also splits out the
> > > > options which are available currently to be built with x86
> > > > and 'make ARCH=arm64' under a shared config.
> > > > 
> > > > Technically xen supports a dom0 kernel and also a guest
> > > > kernel configuration but upon review with the xen team
> > > > since we don't have many dom0 options its best to just
> > > > combine these two into one.
> > > > 
> > > > Cc: Josh Triplett <josh@...htriplett.org>
> > > > Cc: Borislav Petkov <bp@...e.de>
> > > > Cc: Pekka Enberg <penberg@...nel.org>
> > > > Cc: David Rientjes <rientjes@...gle.com>
> > > > Cc: Michal Marek <mmarek@...e.cz>
> > > > Cc: Randy Dunlap <rdunlap@...radead.org>
> > > > Cc: penberg@...nel.org
> > > > Cc: levinsasha928@...il.com
> > > > Cc: mtosatti@...hat.com
> > > > Cc: fengguang.wu@...el.com
> > > > Cc: David Vrabel <david.vrabel@...rix.com>
> > > > Cc: Ian Campbell <Ian.Campbell@...rix.com>
> > > > Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> > > > Cc: xen-devel@...ts.xenproject.org
> > > > Reviewed-by: Josh Triplett <josh@...htriplett.org>
> > > > Signed-off-by: Luis R. Rodriguez <mcgrof@...e.com>
> > > > ---
> > > >  arch/x86/configs/xen.config |  7 +++++++
> > > >  kernel/configs/xen.config   | 30 ++++++++++++++++++++++++++++++
> > > >  scripts/kconfig/Makefile    |  5 +++++
> > > >  3 files changed, 42 insertions(+)
> > > >  create mode 100644 arch/x86/configs/xen.config
> > > >  create mode 100644 kernel/configs/xen.config
> > > > 
> > > > diff --git a/arch/x86/configs/xen.config b/arch/x86/configs/xen.config
> > > > new file mode 100644
> > > > index 0000000..92b8587f
> > > > --- /dev/null
> > > > +++ b/arch/x86/configs/xen.config
> > > > @@ -0,0 +1,7 @@
> > > > +# x86 xen specific config options
> > > > +CONFIG_XEN_PVHVM=y
> > > > +CONFIG_XEN_MAX_DOMAIN_MEMORY=500
> > > > +CONFIG_XEN_SAVE_RESTORE=y
> > > > +# CONFIG_XEN_DEBUG_FS is not set
> > > > +CONFIG_XEN_PVH=y
> > > > +CONFIG_XEN_MCE_LOG=y
> > > > diff --git a/kernel/configs/xen.config b/kernel/configs/xen.config
> > > > new file mode 100644
> > > > index 0000000..d2ec010
> > > > --- /dev/null
> > > > +++ b/kernel/configs/xen.config
> > > > @@ -0,0 +1,30 @@
> > > > +# generic config
> > > > +CONFIG_XEN=y
> > > > +CONFIG_XEN_DOM0=y
> > > > +CONFIG_PCI_XEN=y
> > > 
> > > This shouldn't be here
> > 
> > If PCI is not supported on the arch this won't be selected as kconfig would not
> > allow for it, what would be the issue of keeping it here?
> 
> It looks wrong to me to have an arch specific option here. Moreover many
> ARM boards don't even have PCI, so it looks even worse.
> CONFIG_PCI_XEN is certainly not a generic config option for Xen.

OK fair enough.

> > What xen instances
> > would we not want to have this enabled for and can we instead manage that
> > through Kconfig magic by negating PCI_XEN for it?
> 
> Everything can be fixed via Kconfig magic, the point of this exercise is
> to try to avoid it.
> 
> Are there any cases on x86 where you don't actually want CONFIG_PCI_XEN?

Not that I can think of.

> If not, then why don't just enable it automatically and silently if
> CONFIG_PCI is selected? If we did that, we could avoid specifying it
> here.

CONFIG_PCI_XEN already depends depends on PCI && XEN so I think its safe
to just put it in blindly for x86.

> In any case, given that CONFIG_PCI_XEN is only defined on x86, I think
> it should be in arch/x86/configs/xen.config.

OK!

Will respin.

  Luis
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