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. > I'm not sure how many rules actually care about events for the > subchannel device; the ccw device seems like the more helpful device to > watch out for. I tend to agree, but the problem with vfio-ccw is that (currently) we don't have a ccw device in the host, because we pass-through the subchannel. When we interrogate the subchannel, we do learn if there is a device and if, what is its devno. If I were to run a system with vfio-ccw passthrough, I would want to passthrough the subchannel that talks to the DASD (identified by devno) I need passed through to my guest. Regards, Halil
