linux-kernel - Re: [RFC] /dev/ioasid uAPI proposal

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Date:   Fri, 4 Jun 2021 09:30:54 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Enrico Weigelt, metux IT consult" <lkml@...ux.net>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        David Woodhouse <dwmw2@...radead.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Alex Williamson (alex.williamson@...hat.com)" 
        <alex.williamson@...hat.com>, Jason Wang <jasowang@...hat.com>,
        Eric Auger <eric.auger@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        David Gibson <david@...son.dropbear.id.au>,
        Kirti Wankhede <kwankhede@...dia.com>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [RFC] /dev/ioasid uAPI proposal

On Fri, Jun 04, 2021 at 12:44:28PM +0200, Enrico Weigelt, metux IT consult wrote:
> On 02.06.21 19:24, Jason Gunthorpe wrote:
> 
> Hi,
> 
> >> If I understand this correctly, /dev/ioasid is a kind of "common
> supplier"
> >> to other APIs / devices. Why can't the fd be acquired by the
> >> consumer APIs (eg. kvm, vfio, etc) ?
> >
> > /dev/ioasid would be similar to /dev/vfio, and everything already
> > deals with exposing /dev/vfio and /dev/vfio/N together
> >
> > I don't see it as a problem, just more work.
> 
> One of the problems I'm seeing is in container environments: when
> passing in an vfio device, we now also need to pass in /dev/ioasid,
> thus increasing the complexity in container setup (or orchestration).

Containers already needed to do this today. Container orchestration is
hard.

> And in such scenarios you usually want to pass in one specific device,
> not all of the same class, and usually orchestration shall pick the
> next free one.
> 
> Can we make sure that a process having full access to /dev/ioasid
> while only supposed to have to specific consumer devices, can't do
> any harm (eg. influencing other containers that might use a different
> consumer device) ?

Yes, /dev/ioasid shouldn't do anything unless you have a device to
connect it with. In this way it is probably safe to stuff it into
every container.

> > Having FDs spawn other FDs is pretty ugly, it defeats the "everything
> > is a file" model of UNIX.
> 
> Unfortunately, this is already defeated in many other places :(
> (I'd even claim that ioctls already break it :p)

I think you are reaching a bit :)

> It seems your approach also breaks this, since we now need to open two
> files in order to talk to one device.

It is two devices, thus two files.

Jason