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

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Message-ID: <871r576eqe.fsf@redhat.com>
Date:   Wed, 29 Sep 2021 09:08:25 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        David Gibson <david@...son.dropbear.id.au>,
        "Liu, Yi L" <yi.l.liu@...el.com>
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Subject: RE: [RFC 03/20] vfio: Add vfio_[un]register_device()

On Wed, Sep 29 2021, "Tian, Kevin" <kevin.tian@...el.com> wrote:

>> From: David Gibson <david@...son.dropbear.id.au>
>> Sent: Wednesday, September 29, 2021 10:44 AM
>> 
>> > One alternative option is to arrange device nodes in sub-directories based
>> > on the device type. But doing so also adds one trouble to userspace. The
>> > current vfio uAPI is designed to have the user query device type via
>> > VFIO_DEVICE_GET_INFO after opening the device. With this option the user
>> > instead needs to figure out the device type before opening the device, to
>> > identify the sub-directory.
>> 
>> Wouldn't this be up to the operator / configuration, rather than the
>> actual software though?  I would assume that typically the VFIO
>> program would be pointed at a specific vfio device node file to use,
>> e.g.
>> 	my-vfio-prog -d /dev/vfio/pci/0000:0a:03.1
>> 
>> Or more generally, if you're expecting userspace to know a name in a
>> uniqu pattern, they can equally well know a "type/name" pair.
>> 
>
> You are correct. Currently:
>
> -device, vfio-pci,host=DDDD:BB:DD.F
> -device, vfio-pci,sysfdev=/sys/bus/pci/devices/ DDDD:BB:DD.F
> -device, vfio-platform,sysdev=/sys/bus/platform/devices/PNP0103:00
>
> above is definitely type/name information to find the related node. 
>
> Actually even for Jason's proposal we still need such information to
> identify the sysfs path.
>
> Then I feel type-based sub-directory does work. Adding another link
> to sysfs sounds unnecessary now. But I'm not sure whether we still
> want to create /dev/vfio/devices/vfio0 thing and related udev rule
> thing that you pointed out in another mail.

Still reading through this whole thread, but type-based subdirectories
also make the most sense to me. I don't really see userspace wanting to
grab just any device and then figure out whether it is the device it was
looking for, but rather immediately go to a specific device or at least
a device of a specific type.

Sequentially-numbered devices tend to become really unwieldy in my
experience if you are working on a system with loads of devices.