Date: Fri, 6 Sep 2013 09:57:15 +0300
From: Gleb Natapov <gleb@redhat.com>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Alexey Kardashevskiy <aik@ozlabs.ru>, linuxppc-dev@lists.ozlabs.org,
        David Gibson <david@gibson.dropbear.id.au>,
        Paul Mackerras <paulus@samba.org>, Paolo Bonzini <pbonzini@redhat.com>,
        Alexander Graf <agraf@suse.de>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, kvm-ppc@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCH v9 12/13] KVM: PPC: Add support for IOMMU in-kernel
 handling
Message-ID: <20130906065715.GG13021@redhat.com>
References: <1377679070-3515-1-git-send-email-aik@ozlabs.ru>
 <1377679841-3822-1-git-send-email-aik@ozlabs.ru>
 <20130901120609.GJ22899@redhat.com>
 <52240295.7050608@ozlabs.ru>
 <20130903105315.GY22899@redhat.com>
 <1378353909.4321.126.camel@pasglop>
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On Thu, Sep 05, 2013 at 02:05:09PM +1000, Benjamin Herrenschmidt wrote:
> On Tue, 2013-09-03 at 13:53 +0300, Gleb Natapov wrote:
> > > Or supporting all IOMMU links (and leaving emulated stuff as is) in on
> > > "device" is the last thing I have to do and then you'll ack the patch?
> > > 
> > I am concerned more about API here. Internal implementation details I
> > leave to powerpc experts :)
> 
> So Gleb, I want to step in for a bit here.
> 
> While I understand that the new KVM device API is all nice and shiny and that this
> whole thing should probably have been KVM devices in the first place (had they
> existed or had we been told back then), the point is, the API for handling
> HW IOMMUs that Alexey is trying to add is an extension of an existing mechanism
> used for emulated IOMMUs.
> 
> The internal data structure is shared, and fundamentally, by forcing him to
> use that new KVM device for the "new stuff", we create a oddball API with
> an ioctl for one type of iommu and a KVM device for the other, which makes
> the implementation a complete mess in the kernel (and you should care :-)
> 
Is it unfixable mess? Even if Alexey will do what you suggested earlier?

  - Convert *both* existing TCE objects to the new
      KVM_CREATE_DEVICE, and have some backward compat code for the old one.

The point is implementation usually can be changed, but for API it is
much harder to do so.

> So for something completely new, I would tend to agree with you. However, I
> still think that for this specific case, we should just plonk-in the original
> ioctl proposed by Alexey and be done with it.
> 
Do you think this is the last extension to IOMMU code, or we will see
more and will use same justification to continue adding ioctls?

--
			Gleb.
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