linux-kernel - Re: [PATCH v9 12/13] KVM: PPC: Add support for IOMMU in-kernel handling

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Message-ID: <20130906065715.GG13021@redhat.com>
Date:	Fri, 6 Sep 2013 09:57:15 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Alexey Kardashevskiy <aik@...abs.ru>,
	linuxppc-dev@...ts.ozlabs.org,
	David Gibson <david@...son.dropbear.id.au>,
	Paul Mackerras <paulus@...ba.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Alexander Graf <agraf@...e.de>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, kvm-ppc@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH v9 12/13] KVM: PPC: Add support for IOMMU in-kernel
 handling

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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