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

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Message-ID: <BN9PR11MB543319414486E17EE5BBE7D28CA29@BN9PR11MB5433.namprd11.prod.outlook.com>
Date:   Wed, 22 Sep 2021 01:02:55 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>
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Subject: RE: [RFC 03/20] vfio: Add vfio_[un]register_device()

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Wednesday, September 22, 2021 9:00 AM
> 
> 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.

ah, that's the trick.

> 
> 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
> 

will check. Thanks for suggestion.