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

From: Robin Murphy <robin.murphy@arm.com>
To: Joerg Roedel <joro@8bytes.org>, Jason Gunthorpe <jgg@ziepe.ca>
Cc: "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 16:23:21 +0100	[thread overview]
Message-ID: <131327e3-5066-7a88-5b3c-07013585eb01@arm.com> (raw)
In-Reply-To: <YKKNLrdQ4QjhLrKX@8bytes.org>

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. Using 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.

I seem to recall a non-trivial amount of effort going into the aux 
domain design too, so the promise of replacing it with "a big TBD" just 
because vfio-mdev turned out to be awful hardly fills me with enthusiasm 
either :/

Robin.

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