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

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Message-ID: <b938cc47-029d-ec03-8715-a83d735cf807@ozlabs.ru>
Date:	Fri, 12 Aug 2016 00:04:02 +1000
From:	Alexey Kardashevskiy <aik@...abs.ru>
To:	Cornelia Huck <cornelia.huck@...ibm.com>,
	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, 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 11/08/16 23:19, 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?
> 

I recently posted one for ppc64/book3s:
https://patchwork.ozlabs.org/patch/655305/




-- 
Alexey