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

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Message-ID: <20210922010014.GE327412@nvidia.com>
Date:   Tue, 21 Sep 2021 22:00:14 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     "Liu, Yi L" <yi.l.liu@...el.com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
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        "yi.l.liu@...ux.intel.com" <yi.l.liu@...ux.intel.com>,
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Subject: Re: [RFC 03/20] vfio: Add vfio_[un]register_device()

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.

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