On Wed, 2 Jan 2019 10:53:14 +0100 Cornelia Huck <cohuck@xxxxxxxxxx> wrote: > On Tue, 1 Jan 2019 00:40:19 +0100 > Halil Pasic <pasic@xxxxxxxxxxxxx> wrote: > > As I said, at the moment I don't have a preference regarding the fix, > > partly because I'm not sure if "reading config inside the handler" is OK > > or not. Maybe Connie or Michael can help us here. I'm however sure that > > commit 86a5597 "virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT" > > breaks virtio-balloon with the ccw transport (i.e. effectively breaks > > virtio-balloon on s390): it used to work before and does not work > > after. > > Yes, that's unfortunate. > > > > > AFAICT tweaking the balloon code may be simpler than tweaking the > > virtio-ccw (transport code). ccw_io_helper() relies on getting > > an interrupt when the issued IO is done. If virtio-ccw is buggy, it > > needs to be fixed, but I'm not sure it is. > > I would not call virtio-ccw buggy, but it has some constraints that > virtio-pci apparently doesn't have (and which did not show up so far; > e.g. virtio-blk schedules a work item on config change, so there's no > deadlock there.) > > One way to get out of that constraint (don't interact with the config > space directly in the config changed handler) would be to schedule a > work item in virtio-ccw that calls virtio_config_changed() for the > device. My understanding is that delaying the notification to a work > queue would be fine. Unfortunately, calling virtio_config_changed() from a work item is not enough: That function takes the config_lock, and the virtio-ccw code to get the config both needs to allocate some memory and call schedule :/ The best option really seems to be - have virtio-balloon move the handling of the config change onto a workqueue or something like that, and - document that you cannot read/write the virtio config space from an atomic context Unless someone has a better idea?
