On Tue, 15 Dec 2020 19:13:07 +0100 Cornelia Huck <cohuck@xxxxxxxxxx> wrote: > On Wed, 9 Dec 2020 13:52:03 +0100 > Halil Pasic <pasic@xxxxxxxxxxxxx> wrote: > > > On Tue, 8 Dec 2020 18:30:54 +0100 > > Cornelia Huck <cohuck@xxxxxxxxxx> wrote: > > > > > > > > > > But, the more i look at this patch and discuss on this, i think this is > > > > not complete. > > > > i.e as you know, the main reason for this RFC was the the below thread. > > > > https://marc.info/?l=linux-s390&m=158591045732735&w=2 > > > > We are still not solving the problem that was mentioned in that RFD. > > > > > > > > There are couple of things which we needs to consider here. With this > > > > patch, the uevents > > > > are generated before doing the initialization or before finding the > > > > ccw-device > > > > connected. Which means, the udev-rules have to manage with a > > > > non-initialized setup > > > > compared to the previous version (Version without this patch). As you > > > > mentioned, the > > > > current user-space programs which works with this uevent, especially in > > > > case of vfio-ccw > > > > will have a problem. > > > > > > IIUC, we'll get the "normal" ADD uevent when the subchannel device is > > > registered (i.e. made visible). For the vfio-ccw case, we want the > > > driverctl rule to match in this case, so that the driver override can > > > be set before the subchannel device ends up being bound to the I/O > > > subchannel driver. So I think that removing the suppression is giving > > > us exactly what we want? Modulo any errors in the initialization > > > sequence we might currently have in the css bus code, of course. > > > > > > > I believe, I'm the originator of these concerns, yet I find my > > concern hard to recognize in the comment of Vineeth, so let me > > please try to explain this in a different way. > > > > AFAIK the uevent handling is asynchronous with regards to matching and > > probing, in a sense that there is no synchronization mechanism that > > ensures, the userspace has had the ADD event handled (e.g. > > driver_override set_ before the kernel proceeds with matching and > > probing of the device. Am I wrong about this? > > > > If I'm, with the suppression gone we end up with race, where userspace > > may or may not set driver_override in time. > > > > The man page of driverctl > > (https://manpages.debian.org/testing/driverctl/driverctl.8.en.html) > > claims that: "driverctl integrates with udev to support overriding driver > > selection for both cold- and hotplugged devices from the moment of discovery, ..." > > and "The driver overrides created by driverctl are persistent across > > system reboots by default." > > > > Writing to the driver_override sysfs attribute does not auto-rebind. So > > if we can't ensure being in time to set driver_override for the > > subchannel before the io_subchannel driver binds, then the userspace > > should handle this situation (by unbind and bind) to ensure the > > effectiveness of 'driver override'. I couldn't find that code in > > driverctl, and I assume if we had that, driver override would work > > without this patch. Conny, does that sound about right? > > > > My argument is purely speculative. I didn't try this out, but trying > > stuff out is of limited value with races anyway. Vineeth did you try? > > If not, I could check this out myself some time later. > > Whenever we delegate policy decisions like that to userspace, we'll end > up with uncertainty depending on timing. I don't think that there's any > way around it. (FWIW, driver_override for pci behaves just the same, > unless I misread the code.) > > What removing the uevent suppression does give us is at least a chance > to influence the driver that is being bound and not wait until we have > a fully setup ccw device as a child as well. I finally came around to test this. In my experience driverctl works for subchannels and vfio_ccw without this patch, and continues to work with it. I found the code in driverctl that does the unbind and the implicit bind (via drivers_probe after after driver_override was set). So now I have to ask, how exactly was the original problem diagnosed? In https://marc.info/?l=linux-s390&m=158591045732735&w=2 there is a paragraph like: """ So while there's definitely a good reason for wanting to delay uevents, it is also introducing problems. One is udev rules for subchannels that are supposed to do something before a driver binds (e.g. setting driver_override to bind an I/O subchannel to vfio_ccw instead of io_subchannel) are not effective, as the ADD uevent will only be generated when the io_subchannel driver is already done with doing all setup. Another one is that only the ADD uevent is generated after uevent suppression is lifted; any other uevents that might have been generated are lost. """ This is not how driverclt works! I.e. it deals with the situation that the I/O subchannel was already bound to the io_subchannel driver at the time the udev rule installed by driverctl activates (via the mechanism I described above). Regards, Halil
