From mboxrd@z Thu Jan  1 00:00:00 1970
From: jean-philippe.brucker@arm.com (Jean-Philippe Brucker)
Date: Thu, 24 May 2018 16:04:39 +0100
Subject: [PATCH v2 03/40] iommu/sva: Manage process address spaces
In-Reply-To: <20180524115039.GA10260@apalos>
References: <20180511190641.23008-1-jean-philippe.brucker@arm.com>
 <20180511190641.23008-4-jean-philippe.brucker@arm.com>
 <20180516163117.622693ea@jacob-builder>
 <de478769-9f7a-d40b-a55e-e2c63ad883e8@arm.com>
 <20180522094334.71f0e36b@jacob-builder>
 <f73b4a0e-669e-8483-88d7-1b2c8a2b9934@arm.com>
 <20180524115039.GA10260@apalos>
Message-ID: <19e82a74-429a-3f86-119e-32b12082d0ff@arm.com>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

On 24/05/18 12:50, Ilias Apalodimas wrote:
>> Interesting, I hadn't thought about this use-case before. At first I
>> thought you were talking about mdev devices assigned to VMs, but I think
>> you're referring to mdevs assigned to userspace drivers instead? Out of
>> curiosity, is it only theoretical or does someone actually need this?
> 
> There has been some non upstreamed efforts to have mdev and produce userspace
> drivers. Huawei is using it on what they call "wrapdrive" for crypto devices and
> we did a proof of concept for ethernet interfaces. At the time we choose not to
> involve the IOMMU for the reason you mentioned, but having it there would be
> good.

I'm guessing there were good reasons to do it that way but I wonder, is
it not simpler to just have the kernel driver create a /dev/foo, with a
standard ioctl/mmap/poll interface? Here VFIO adds a layer of
indirection, and since the mediating driver has to implement these
operations already, what is gained?

Thanks,
Jean