linux-kernel - Re: [PATCH 3/4] KVM: PPC: Add support for IOMMU in-kernel handling

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Date:	Tue, 18 Jun 2013 14:38:55 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	Alexey Kardashevskiy <aik@...abs.ru>,
	linuxppc-dev@...ts.ozlabs.org,
	David Gibson <david@...son.dropbear.id.au>,
	Alexander Graf <agraf@...e.de>,
	Paul Mackerras <paulus@...ba.org>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, kvm-ppc@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH 3/4] KVM: PPC: Add support for IOMMU in-kernel handling

On Mon, 2013-06-17 at 20:32 -0600, Alex Williamson wrote:

> Right, we don't want to create dependencies across modules.  I don't
> have a vision for how this should work.  This is effectively a complete
> side-band to vfio, so we're really just dealing in the iommu group
> space.  Maybe there needs to be some kind of registration of ownership
> for the group using some kind of token.  It would need to include some
> kind of notification when that ownership ends.  That might also be a
> convenient tag to toggle driver probing off for devices in the group.
> Other ideas?  Thanks,

All of that smells nasty like it will need a pile of bloody
infrastructure.... which makes me think it's too complicated and not the
right approach.

How does access control work today on x86/VFIO ? Can you give me a bit
more details ? I didn't get a good grasp in your previous email....

>From the look of it, the VFIO file descriptor is what has the "access
control" to the underlying iommu, is this right ? So we somewhat need to
transfer (or copy) that ownership from the VFIO fd to the KVM VM.

I don't see a way to do that without some cross-layering here...

Rusty, are you aware of some kernel mechanism we can use for that ?

Cheers,
Ben.


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