linux-kernel - Re: [RFC 03/20] vfio: Add vfio_[un]register

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Date:   Thu, 23 Sep 2021 08:44:59 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Eric Auger <eric.auger@...hat.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "hch@....de" <hch@....de>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "jean-philippe@...aro.org" <jean-philippe@...aro.org>,
        "parav@...lanox.com" <parav@...lanox.com>,
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        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "lushenming@...wei.com" <lushenming@...wei.com>,
        "corbet@....net" <corbet@....net>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "yi.l.liu@...ux.intel.com" <yi.l.liu@...ux.intel.com>,
        "Tian, Jun J" <jun.j.tian@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "jacob.jun.pan@...ux.intel.com" <jacob.jun.pan@...ux.intel.com>,
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        "david@...son.dropbear.id.au" <david@...son.dropbear.id.au>,
        "nicolinc@...dia.com" <nicolinc@...dia.com>
Subject: Re: [RFC 03/20] vfio: Add vfio_[un]register_device()

On Thu, Sep 23, 2021 at 09:25:27AM +0200, Eric Auger wrote:
> Hi,
> 
> On 9/22/21 3:00 AM, Jason Gunthorpe wrote:
> > On Wed, Sep 22, 2021 at 12:54:02AM +0000, Tian, Kevin wrote:
> >>> From: Jason Gunthorpe <jgg@...dia.com>
> >>> Sent: Wednesday, September 22, 2021 12:01 AM
> >>>
> >>>>  One open about how to organize the device nodes under
> >>> /dev/vfio/devices/.
> >>>> This RFC adopts a simple policy by keeping a flat layout with mixed
> >>> devname
> >>>> from all kinds of devices. The prerequisite of this model is that devnames
> >>>> from different bus types are unique formats:
> >>> This isn't reliable, the devname should just be vfio0, vfio1, etc
> >>>
> >>> The userspace can learn the correct major/minor by inspecting the
> >>> sysfs.
> >>>
> >>> This whole concept should disappear into the prior patch that adds the
> >>> struct device in the first place, and I think most of the code here
> >>> can be deleted once the struct device is used properly.
> >>>
> >> Can you help elaborate above flow? This is one area where we need
> >> more guidance.
> >>
> >> When Qemu accepts an option "-device vfio-pci,host=DDDD:BB:DD.F",
> >> how does Qemu identify which vifo0/1/... is associated with the specified 
> >> DDDD:BB:DD.F? 
> > When done properly in the kernel the file:
> >
> > /sys/bus/pci/devices/DDDD:BB:DD.F/vfio/vfioX/dev
> >
> > Will contain the major:minor of the VFIO device.
> >
> > Userspace then opens the /dev/vfio/devices/vfioX and checks with fstat
> > that the major:minor matches.
> >
> > in the above pattern "pci" and "DDDD:BB:DD.FF" are the arguments passed
> > to qemu.
> I guess this would be the same for platform devices, for instance
> /sys/bus/platform/devices/AMDI8001:01/vfio/vfioX/dev, right?

Yes, it is the general driver core pattern for creating cdevs below a
parent device

Jason