RE: mechanism to allow a driver to bind to any device

[Stuart Yoder]

> -----Original Message-----
> From: Alex Williamson [mailto:alex.williamson@xxxxxxxxxx]
> Sent: Wednesday, March 26, 2014 5:09 PM
> To: Alexander Graf
> Cc: kvm@xxxxxxxxxxxxxxx; jan.kiszka@xxxxxxxxxxx; will.deacon@xxxxxxx;
> Yoder Stuart-B08248; a.rigo@xxxxxxxxxxxxxxxxxxxxxx; Michal Hocko; Wood
> Scott-B07421; Sethi Varun-B16395; kvmarm@xxxxxxxxxxxxxxxxxxxxx; Rafael J.
> Wysocki; Guenter Roeck; Dmitry Kasatkin; Tejun Heo; Bjorn Helgaas;
> Antonios Motakis; tech@xxxxxxxxxxxxxxxxxxxxxx; Toshi Kani; Greg KH;
> linux-kernel@xxxxxxxxxxxxxxx; iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx; Joe
> Perches; christoffer.dall@xxxxxxxxxx
> Subject: Re: mechanism to allow a driver to bind to any device
> 
> On Wed, 2014-03-26 at 10:21 -0600, Alex Williamson wrote:
> > On Wed, 2014-03-26 at 23:06 +0800, Alexander Graf wrote:
> > >
> > > > Am 26.03.2014 um 22:40 schrieb Konrad Rzeszutek Wilk
> <konrad.wilk@xxxxxxxxxx>:
> > > >
> > > >> On Wed, Mar 26, 2014 at 01:40:32AM +0000, Stuart Yoder wrote:
> > > >> Hi Greg,
> > > >>
> > > >> We (Linaro, Freescale, Virtual Open Systems) are trying get an
> issue
> > > >> closed that has been perculating for a while around creating a
> mechanism
> > > >> that will allow kernel drivers like vfio can bind to devices of
> any type.
> > > >>
> > > >> This thread with you:
> > > >> http://www.spinics.net/lists/kvm-arm/msg08370.html
> > > >> ...seems to have died out, so am trying to get your response
> > > >> and will summarize again.  Vfio drivers in the kernel (regardless
> of
> > > >> bus type) need to bind to devices of any type.  The driver's
> function
> > > >> is to simply export hardware resources of any type to user space.
> > > >>
> > > >> There are several approaches that have been proposed:
> > > >
> > > > You seem to have missed the one I proposed.
> > > >>
> > > >>   1.  new_id -- (current approach) the user explicitly registers
> > > >>       each new device type with the vfio driver using the new_id
> > > >>       mechanism.
> > > >>
> > > >>       Problem: multiple drivers will be resident that handle the
> > > >>       same device type...and there is nothing user space hotplug
> > > >>       infrastructure can do to help.
> > > >>
> > > >>   2.  "any id" -- the vfio driver could specify a wildcard match
> > > >>       of some kind in its ID match table which would allow it to
> > > >>       match and bind to any possible device id.  However,
> > > >>       we don't want the vfio driver grabbing _all_ devices...just
> the ones we
> > > >>       explicitly want to pass to user space.
> > > >>
> > > >>       The proposed patch to support this was to create a new flag
> > > >>       "sysfs_bind_only" in struct device_driver.  When this flag
> > > >>       is set, the driver can only bind to devices via the sysfs
> > > >>       bind file.  This would allow the wildcard match to work.
> > > >>
> > > >>       Patch is here:
> > > >>       https://lkml.org/lkml/2013/12/3/253
> > > >>
> > > >>   3.  "Driver initiated explicit bind" -- with this approach the
> > > >>       vfio driver would create a private 'bind' sysfs object
> > > >>       and the user would echo the requested device into it:
> > > >>
> > > >>       echo 0001:03:00.0 > /sys/bus/pci/drivers/vfio-pci/vfio_bind
> > > >>
> > > >>       In order to make that work, the driver would need to call
> > > >>       driver_probe_device() and thus we need this patch:
> > > >>       https://lkml.org/lkml/2014/2/8/175
> > > >
> > > > 4). Use the 'unbind' (from the original device) and 'bind' to vfio
> driver.
> > >
> > > This is approach 2, no?
> > >
> > > >
> > > > Which I think is what is currently being done. Why is that not
> sufficient?
> > >
> > > How would 'bind to vfio driver' look like?
> > >
> > > > The only thing I see in the URL is " That works, but it is ugly."
> > > > There is some mention of race but I don't see how - if you do the
> 'unbind'
> > > > on the original driver and then bind the BDF to the VFIO how would
> you get
> > > > a race?
> > >
> > > Typically on PCI, you do a
> > >
> > >   - add wildcard (pci id) match to vfio driver
> > >   - unbind driver
> > >   -> reprobe
> > >   -> device attaches to vfio driver because it is the least recent
> match
> > >   - remove wildcard match from vfio driver
> > >
> > > If in between you hotplug add a card of the same type, it gets
> attached to vfio - even though the logical "default driver" would be the
> device specific driver.
> >
> > I've mentioned drivers_autoprobe in the past, but I'm not sure we're
> > really factoring it into the discussion.  drivers_autoprobe allows us
> to
> > toggle two points:
> >
> > a) When a new device is added whether we automatically give drivers a
> > try at binding to it
> >
> > b) When a new driver is added whether it gets to try to bind to
> anything
> > in the system
> >
> > So we do have a mechanism to avoid the race, but the problem is that it
> > becomes the responsibility of userspace to:
> >
> > 1) turn off drivers_autoprobe
> > 2) unbind/new_id/bind/remove_id
> > 3) turn on drivers_autoprobe
> > 4) call drivers_probe for anything added between 1) & 3)
> >
> > Is the question about the ugliness of the current solution whether it's
> > unreasonable to ask userspace to do this?
> >
> > What we seem to be asking for above is more like an autoprobe flag per
> > driver where there's some way for this special driver to opt out of
> auto
> > probing.  Option 2. in Stuart's list does this by short-cutting ID
> > matching so that a "match" is only found when using the sysfs bind
> path,
> > option 3. enables a way for a driver to expose their own sysfs entry
> > point for binding.  The latter feels particularly chaotic since drivers
> > get to make-up their own bind mechanism.
> >
> > Another twist I'll throw in is that devices can be hot added to IOMMU
> > groups that are in-use by userspace.  When that happens we'd like to be
> > able to disable driver autoprobe of the device to avoid a host driver
> > automatically binding to the device.  I wonder if instead of looking at
> > the problem from the driver perspective, if we were to instead look at
> > it from the device perspective if we might find a solution that would
> > address both.  For instance, if devices had a driver_probe_id property
> > that was by default set to their bus specific ID match ("$VENDOR
> > $DEVICE" on PCI) could we use that to write new match IDs so that a
> > device could only bind to a given driver?  Effectively we could then
> > bind either using the current method of adding to the list of IDs a
> > driver will match of changing the ID that a device would match.  Does
> > that get us anywhere?  Thanks,
> 
> Here's one way this might work for PCI; note that we can do this
> entirely in the bus driver for PCI.  Bind/unbind would go like this:
> 
> # bind device to vfio-pci
> echo vfio-pci > /sys/bus/pci/devices/0000\:03\:00.0/preferred_driver
> echo 0000:03:00.0 > /sys/bus/pci/devices/0000\:03\:00.0/driver/unbind
> echo 0000:03:00.0 > /sys/bus/pci/drivers_probe
> 
> # bind device back to host driver
> echo > /sys/bus/pci/devices/0000\:03\:00.0/preferred_driver
> echo 0000:03:00.0 > /sys/bus/pci/devices/0000\:03\:00.0/driver/unbind
> echo 0000:03:00.0 > /sys/bus/pci/drivers_probe
> 
> When preferred_driver is set for a device it will match and bind only to
> a driver with a matching name.  This also means we can write random
> strings here to avoid a device being bound to any driver if we want.
> 
> In the example patch below I've put the preferred_driver in the struct
> pci_dev, but if this mechanism were adopted by multiple devices perhaps
> we could add it to struct device.  Would something like this work for
> platform devices?
> 
> Note 1, the below is just the core PCI driver change to support this,
> there's some trivial collateral damage from changing an exported
> function not shown here for brevity.
> 
> Note 2, PCI passes a struct pci_device_id to the driver probe function
> which would be NULL in the preferred driver case of the example below.
> We'd need to dynamically create one of these when calling the probe
> function to make this practical for drivers that use that data.  Thanks,

The paradigm of telling the device what the preferred driver is feels
more awkward to me than a sysfs flag for the driver to opt out of
auto-probing...but at this point if there is consensus that the
preferred_driver approach will be accepted upstream, I'm ok with it.
It think it works.

However, I am concerned about getting 'preferred driver' accepted
into the kernel and it's not immediately obvious to me how it is more
palatable than the 'opt out of auto-probe' approaches that were
proposed previously.

I also, was at the point where I thought we should perhaps just
go with current mechanisms and implement new_id for the platform
bus...but Greg's recent response is 'platform devices suck' and it sounds
like he would reject a new_id patch for the platform bus.  So it kind
of feels like we are stuck.

Thanks,
Stuart