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

david
2021-09-29T03:28:07Z

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On Tue, Sep 21, 2021 at 10:00:14PM -0300, 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 thought part of the appeal of the device centric model was less
grovelling around in sysfs for information. Using type/address
directly in /dev seems simpler than having to dig around matching
things here.

Note that this doesn't have to be done in kernel: you could have the
kernel just call them /dev/vfio/devices/vfio0, ... but add udev rules
that create symlinks from say /dev/vfio/pci/DDDD:BB:SS.F - >
../devices/vfioXX based on the sysfs information.

>
> 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
>

--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
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