There is a situation where removing all the paths from a device connected via mdev and vfio-ccw can cause some difficulty. Using the "chchp -c 0 xx" command to all paths will cause the device to be removed from the configuration, and any guest filesystem that is relying on that device will encounter errors. Interestingly, the last chchp command will actually fail to return to a shell prompt, and subsequent commands (another chchp to bring the paths back online, chzdev, etc.) will also hang because of the outstanding chchp. The last chchp command drives to vfio_ccw_sch_remove() for every affected mediated device, and ultimately enters an infinite loop in vfio_del_group_dev(). This loop is broken when the guest goes away, which in this case doesn't occur until the guest is shutdown. This drives vfio_ccw_mdev_release() and thus vfio_device_release() to wake up the vfio_del_group_dev() thread. There is also a callback mechanism called "request" to ask a driver (and perhaps user) to release the device, but this is not implemented for mdev. So this adds one to that point, and then wire it to vfio-ccw to pass it along to userspace. This will gracefully drive the unplug code, and everything behaves nicely. Despite the testing that was occurring, this doesn't appear related to the vfio-ccw channel path handling code. I can reproduce this with an older kernel/QEMU, which makes sense because the above behavior is driven from the subchannel event codepaths and not the chpid events. Because of that, I didn't flag anything with a Fixes tag, since it's seemingly been this way forever. RFC->V2: - Patch 1 - Added a message when registering a device without a request callback - Changed the "if(!callback) return" to "if(callback) do" layout - Removed "unlikely" from "if(callback)" logic - Clarified some wording in the device ops struct commentary - Patch 2 - Added Conny's r-b Eric Farman (2): vfio-mdev: Wire in a request handler for mdev parent vfio-ccw: Wire in the request callback drivers/s390/cio/vfio_ccw_ops.c | 26 ++++++++++++++++++++++++++ drivers/s390/cio/vfio_ccw_private.h | 4 ++++ drivers/vfio/mdev/mdev_core.c | 4 ++++ drivers/vfio/mdev/vfio_mdev.c | 10 ++++++++++ include/linux/mdev.h | 4 ++++ include/uapi/linux/vfio.h | 1 + 6 files changed, 49 insertions(+) -- 2.17.1
