linux-kernel - Re: [PATCH] vfio: Enable VFIO device for powerpc

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Message-ID: <20160812064631.GW16493@voom.fritz.box>
Date:	Fri, 12 Aug 2016 16:46:31 +1000
From:	David Gibson <david@...son.dropbear.id.au>
To:	Cornelia Huck <cornelia.huck@...ibm.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	Paul Mackerras <paulus@...abs.org>,
	Alexander Graf <agraf@...e.de>, alex.williamson@...hat.com,
	aik@...abs.ru, mdroth@...ux.vnet.ibm.com, benh@...nel.crashing.org,
	mpe@...erman.id.au, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, kvm-ppc <kvm-ppc@...r.kernel.org>
Subject: Re: [PATCH] vfio: Enable VFIO device for powerpc

On Thu, Aug 11, 2016 at 03:19:57PM +0200, Cornelia Huck wrote:
> On Thu, 11 Aug 2016 14:57:24 +0200
> Paolo Bonzini <pbonzini@...hat.com> wrote:
> 
> > On 26/08/2015 20:54, Paul Mackerras wrote:
> > > On Wed, Aug 26, 2015 at 11:34:26AM +0200, Alexander Graf wrote:
> > >> On 13.08.15 03:15, David Gibson wrote:
> > >>> ec53500f "kvm: Add VFIO device" added a special KVM pseudo-device which is
> > >>> used to handle any necessary interactions between KVM and VFIO.
> > >>>
> > >>> Currently that device is built on x86 and ARM, but not powerpc, although
> > >>> powerpc does support both KVM and VFIO.  This makes things awkward in
> > >>> userspace
> > >>>
> > >>> Currently qemu prints an alarming error message if you attempt to use VFIO
> > >>> and it can't initialize the KVM VFIO device.  We don't want to remove the
> > >>> warning, because lack of the KVM VFIO device could mean coherency problems
> > >>> on x86.  On powerpc, however, the error is harmless but looks disturbing,
> > >>> and a test based on host architecture in qemu would be ugly, and break if
> > >>> we do need the KVM VFIO device for something important in future.
> > >>>
> > >>> There's nothing preventing the KVM VFIO device from being built for
> > >>> powerpc, so this patch turns it on.  It won't actually do anything, since
> > >>> we don't define any of the arch_*() hooks, but it will make qemu happy and
> > >>> we can extend it in future if we need to.
> > >>>
> > >>> Signed-off-by: David Gibson <david@...son.dropbear.id.au>
> > >>> Reviewed-by: Eric Auger <eric.auger@...aro.org>
> > 
> > This patch (commit 178a787502123) did not select CONFIG_KVM_VFIO, so the
> > patch did nothing---except causing build failures which I fixed in
> > commit 0af574be32cdd ("KVM: PPC: do not compile in vfio.o
> > unconditionally", 2016-03-21) by making the patch a total no-op.
> > 
> > Is KVM_VFIO really needed, and if so can this patch be fixed?
> 
> FWIW, we enabled building vfio.o on s390 in 14b0b4a ("KVM: s390: Enable
> the KVM-VFIO device") with the rationale "while we don't need it, be
> like everybody else".
> 
> Should powerpc (and every other architecture supporting kvm and vfio)
> select KVM_VFIO so that really everybody does the same thing?

Yes, I think it should.  That was my intention when I sent that patch
- I just messed it up.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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