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

From: Jason Gunthorpe <jgg@ziepe.ca>
To: Robin Murphy <robin.murphy@arm.com>
Cc: Joerg Roedel <joro@8bytes.org>,
	"Tian, Kevin" <kevin.tian@intel.com>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	Will Deacon <will@kernel.org>,
	Kirti Wankhede <kwankhede@nvidia.com>,
	"iommu@lists.linux-foundation.org"
	<iommu@lists.linux-foundation.org>,
	Alex Williamson <alex.williamson@redhat.com>,
	David Woodhouse <dwmw2@infradead.org>,
	Christoph Hellwig <hch@lst.de>,
	"linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH 3/6] vfio: remove the unused mdev iommu hook
Date: Wed, 19 May 2021 15:06:35 -0300	[thread overview]
Message-ID: <20210519180635.GT1096940@ziepe.ca> (raw)
In-Reply-To: <131327e3-5066-7a88-5b3c-07013585eb01@arm.com>

On Wed, May 19, 2021 at 04:23:21PM +0100, Robin Murphy wrote:
> On 2021-05-17 16:35, Joerg Roedel wrote:
> > On Mon, May 17, 2021 at 10:35:00AM -0300, Jason Gunthorpe wrote:
> > > Well, I'm sorry, but there is a huge other thread talking about the
> > > IOASID design in great detail and why this is all needed. Jumping into
> > > this thread without context and basically rejecting all the
> > > conclusions that were reached over the last several weeks is really
> > > not helpful - especially since your objection is not technical.
> > > 
> > > I think you should wait for Intel to put together the /dev/ioasid uAPI
> > > proposal and the example use cases it should address then you can give
> > > feedback there, with proper context.
> > 
> > Yes, I think the next step is that someone who read the whole thread
> > writes up the conclusions and a rough /dev/ioasid API proposal, also
> > mentioning the use-cases it addresses. Based on that we can discuss the
> > implications this needs to have for IOMMU-API and code.
> > 
> >  From the use-cases I know the mdev concept is just fine. But if there is
> > a more generic one we can talk about it.
> 
> Just to add another voice here, I have some colleagues working on drivers
> where they want to use SMMU Substream IDs for a single hardware block to
> operate on multiple iommu_domains managed entirely within the
> kernel.

If it is entirely within the kernel I'm confused how mdev gets
involved? mdev is only for vfio which is userspace.

In any event, if you are kernel only it is not quite as big a deal to
create what you need. A 'substream domain' disconnected from the
struct device is not unreasonable.

> an mdev-like approach with aux domains is pretty much the ideal fit for this
> use-case, while all the IOASID discussion appears centred on SVA and
> userspace interfaces, and as such barely relevant if at all.

/dev/ioasid is centered on userspace, but usually the way this works
is a user API like that will be close to a mirror kernel
implementation if someone needs such a thing.

Jason