Hello,
--
Antonios Motakis
Virtual Open Systems
On Wed, Mar 26, 2014 at 2:40 AM, Stuart Yoder <stuart.yoder@xxxxxxxxxxxxx> 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:
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.
Of note is that new_id doesn't work particularly well for platform devices. Before trying any of the other two solutions, a nasty hack was applied on the device tree used with the system in order to let vfio-platform match with a specific device, which is certainly not an acceptable solution.
Implementing wildcard matching quickly reveals the problems mentioned, which motivates the other two approaches.
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
Would like your comment on these options-- option #3 is preferred
and is literally a 2 line patch.
I would definitely agree with approach #3, for which Kim has already provided a patch. Not having this would make using VFIO with platform devices really inelegant and strange.
Thanks,
Stuart
--
Antonios Motakis
Virtual Open Systems
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