On Wed, 2 Jan 2019 14:23:38 +0100 Cornelia Huck <cohuck@xxxxxxxxxx> wrote: > 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? > I wonder, would making config_lock a mutex suffice? I've already hinted that a virtio-balloon side fix is probably the simpler variant. I agree, let's fix the regression first, and think about wether to teach virtio-ccw to allow config manipulation from virtio_config_changed() or not later. Regards, Halil
