Re: [PATCH v2 03/40] iommu/sva: Manage process address spaces

Ilias Apalodimas
2018-05-25T06:33:11Z

On Thu, May 24, 2018 at 04:04:39PM +0100, Jean-Philippe Brucker wrote:
> 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?
The best reason i can come up with is "common code". You already have one API
doing that for you so we replicate it in a /dev file?
The mdev approach still needs extentions to support what we tried to do (i.e
mdev bus might need yo have access on iommu_ops), but as far as i undestand it's
a possible case.
>
> Thanks,
> Jean