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

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Date:	Thu, 11 Aug 2016 15:19:57 +0200
From:	Cornelia Huck <cornelia.huck@...ibm.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Paul Mackerras <paulus@...abs.org>, Alexander Graf <agraf@...e.de>,
	David Gibson <david@...son.dropbear.id.au>,
	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, 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?