Re: [RFC 03/20] vfio: Add vfio_[un]register_device()

Eric Auger
2021-09-23T07:25:43Z

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@nvidia.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?

Thanks

Eric
>
> You can look at this for some general over engineered code to handle
> opening from a sysfs handle like above:
>
> https://github.com/linux-rdma/rdma-core/blob/master/util/open_cdev.c
>
> Jason