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

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Message-ID: <7d717ad0-fb9b-2af0-7818-147dc5d21373@redhat.com>
Date:   Thu, 23 Sep 2021 09:25:27 +0200
From:   Eric Auger <eric.auger@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        "Tian, Kevin" <kevin.tian@...el.com>
Cc:     "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>,
        "lkml@...ux.net" <lkml@...ux.net>,
        "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>,
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Subject: Re: [RFC 03/20] vfio: Add vfio_[un]register_device()

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?

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
>