On Wed, Mar 26, 2014 at 10:49:52AM -0600, Alex Williamson wrote: > On Wed, 2014-03-26 at 12:32 -0400, Konrad Rzeszutek Wilk wrote: > > On Wed, Mar 26, 2014 at 10:21:02AM -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, > > > > The other option for this is to having some sort of priority on the > > device probing with hotplugging. > > > > That is you can could do the following: > > > > 1) add the device vendor/model in vfio > > 2) unbind the BDF from the original driver. > > 3) hotplug happens - any new device that has the device vendor/model gets > > owned by vfio instead of the original device. > > This doesn't help the device-added-to-inuse-group problem though because > we have no idea if the new device would have the same vendor/model as > other devices in the group. By making the device probe ID modifiable, Um, you add a hotplugged PCI device in a group that is in usage? > vfio can watch the IOMMU group notifiers and change the probe ID of new Ewwww. > devices to either prevent the host driver from claiming them or to allow > vfio to claim them. At the same time we change the problem from "this > driver can attach to this kind of device" to "this device can attach to > that driver", which also solves Stuart's problem. Thanks, > > Alex > > > 4). bind the BDF to the vfio. > > > > Granted that is a bit silly too - as the admin might want to have the new > > hotplugged device be owned by the native driver. > > > > In which case, why not just switch out from using device vendor/model > > to just using BDF values? Which would still solve the problem. The user-space would just have to reassign the device to the vfio group. > > > > > > Alex > > > > > > _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
