Re: [PATCH 3/6] vfio: remove the unused mdev iommu hook

From: Joerg Roedel <joro@8bytes.org>
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: "Tian, Kevin" <kevin.tian@intel.com>,
	Christoph Hellwig <hch@lst.de>,
	Alex Williamson <alex.williamson@redhat.com>,
	David Woodhouse <dwmw2@infradead.org>,
	Lu Baolu <baolu.lu@linux.intel.com>,
	Will Deacon <will@kernel.org>,
	Kirti Wankhede <kwankhede@nvidia.com>,
	"linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
	"iommu@lists.linux-foundation.org"
	<iommu@lists.linux-foundation.org>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>
Subject: Re: [PATCH 3/6] vfio: remove the unused mdev iommu hook
Date: Mon, 17 May 2021 14:53:16 +0200	[thread overview]
Message-ID: <YKJnPGonR+d8rbu/@8bytes.org> (raw)
In-Reply-To: <20210517123010.GO1096940@ziepe.ca>

On Mon, May 17, 2021 at 09:30:10AM -0300, Jason Gunthorpe wrote:
> On Mon, May 17, 2021 at 02:22:06PM +0200, Joerg Roedel wrote:
> > Yes, I know, We have the AUX-domain specific functions now, but I
> > suggested a while back to turn the mdev code into its own bus
> > implementation and let the IOMMU driver deal with whether it has an mdev
> > or a pdev. When that is done the aux-specific functions can go away.
> 
> Yuk, no.
> 
> PASID is not connected to the driver model. It is inherently wrong to
> suggest this.

There will be no drivers for that, but an mdev is a container for
resources of a physical device which can be assigned to a virtual
machine or a user-space process. In this way it fits very well into the
existing abstractions.

> PASID is a property of a PCI device and any PCI device driver should
> be able to spawn PASIDs without silly restrictions.

Some points here:

	1) The concept of PASIDs is not limited to PCI devices.

	2) An mdev is not a restriction. It is an abstraction with which
	   other parts of the kernel can work.

> Fixing the IOMMU API is clearly needed here to get a clean PASID
> implementation in the kernel.

You still have to convince me that this is needed and a good idea. By
now I am not even remotely convinced and putting words like 'fixing',
'clearly' and 'silly' in a sentence is by far not enough for this to
happen.

> We aren't talking about mm_struct.

Passing an mm_struct to a device is another use-case for PASIDs, you
should keep that in mind. The mdev abstraction was made for a different
use-case.

To be clear, I agree that the aux-domain specifics should be removed
from the IOMMU-API, but the mdev-abstraction itself still makes sense.

> As above a domain isn't an IOASID, it only does a few things an IOASID
> can do.

For example?

Regards,

	Joerg