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

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Message-ID: <YVPTdqWw6or3mK/h@yekko>
Date:   Wed, 29 Sep 2021 12:46:14 +1000
From:   "david@...son.dropbear.id.au" <david@...son.dropbear.id.au>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
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Subject: Re: [RFC 03/20] vfio: Add vfio_[un]register_device()

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@...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 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_!
http://www.ozlabs.org/~dgibson

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