> -----Original Message----- > From: Alex Williamson [mailto:alex.william...@redhat.com] > Sent: Wednesday, March 26, 2014 5:09 PM > To: Alexander Graf > Cc: k...@vger.kernel.org; jan.kis...@siemens.com; will.dea...@arm.com; > Yoder Stuart-B08248; a.r...@virtualopensystems.com; Michal Hocko; Wood > Scott-B07421; Sethi Varun-B16395; kvm...@lists.cs.columbia.edu; Rafael J. > Wysocki; Guenter Roeck; Dmitry Kasatkin; Tejun Heo; Bjorn Helgaas; > Antonios Motakis; t...@virtualopensystems.com; Toshi Kani; Greg KH; > linux-kernel@vger.kernel.org; io...@lists.linux-foundation.org; Joe > Perches; christoffer.d...@linaro.org > 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.w...@oracle.com>: > > > > > > > >> 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
