Prior to the virtio-ccw-2.7 machine (and commit 2a79eb1a), our virtio devices residing under the virtual-css bus do not have qdev_path based migration stream identifiers (because their qdev_path is NULL). The ids are instead generated when the device is registered as a composition of the so called idstr, which takes the vmsd name as its value, and an instance_id, which is which is calculated as a maximal instance_id registered with the same idstr plus one, or zero (if none was registered previously).
That means, under certain circumstances, one device might try, and even succeed, to load the state of a different device. This can lead to trouble. Let us fail the migration if the above problem is detected during load. How to reproduce the problem: 1) start qemu-system-s390x making sure you have the following devices defined on your command line: -device virtio-rng-ccw,id=rng1,devno=fe.0.0001 -device virtio-rng-ccw,id=rng2,devno=fe.0.0002 2) detach the devices and reattach in reverse order using the monitor: (qemu) device_del rng1 (qemu) device_del rng2 (qemu) device_add virtio-rng-ccw,id=rng2,devno=fe.0.0002 (qemu) device_add virtio-rng-ccw,id=rng1,devno=fe.0.0001 3) save the state of the vm into a temporary file and quit QEMU: (qemu) migrate "exec:gzip -c > /tmp/tmp_vmstate.gz" (qemu) q 4) use your command line from step 1 with -incoming "exec:gzip -c -d /tmp/tmp_vmstate.gz" appended to reproduce the problem (while trying to to load the saved vm) CC: qemu-sta...@nongnu.org Signed-off-by: Halil Pasic <pa...@linux.vnet.ibm.com> Reviewed-by: Dong Jia Shi <bjsdj...@linux.vnet.ibm.com> --- Hi! I also wonder what is the best way to do this with vmstate. I know there are VMSTATE_*_EQUAL macros for integers, and I have partially modelled my patch after that, but there we only get a != b as error message, which is satisfactory for detecting bugs which are supposed to get fixed. In this particular case having a verbose error message should be really helpful and thus important. I'm asking because I'm currently working on a vmstate conversion of the s390x css and virtio-ccw stuff (find my latest patch set here https://lists.gnu.org/archive/html/qemu-devel/2017-05/msg01364.html). Regards, Halil --- hw/s390x/css.c | 14 ++++++++++++++ hw/s390x/virtio-ccw.c | 6 +++++- 2 files changed, 19 insertions(+), 1 deletion(-) diff --git a/hw/s390x/css.c b/hw/s390x/css.c index 15c4f4b..6cff3a3 100644 --- a/hw/s390x/css.c +++ b/hw/s390x/css.c @@ -14,6 +14,7 @@ #include "qapi/visitor.h" #include "hw/qdev.h" #include "qemu/bitops.h" +#include "qemu/error-report.h" #include "exec/address-spaces.h" #include "cpu.h" #include "hw/s390x/ioinst.h" @@ -1721,13 +1722,26 @@ void subch_device_save(SubchDev *s, QEMUFile *f) int subch_device_load(SubchDev *s, QEMUFile *f) { SubchDev *old_s; + Error *err = NULL; uint16_t old_schid = s->schid; + uint16_t old_devno = s->devno; int i; s->cssid = qemu_get_byte(f); s->ssid = qemu_get_byte(f); s->schid = qemu_get_be16(f); s->devno = qemu_get_be16(f); + if (s->devno != old_devno) { + /* Only possible if machine < 2.7 (no css_dev_path) */ + + error_setg(&err, "%x != %x", old_devno, s->devno); + error_append_hint(&err, "Devno mismatch, tried to load wrong section!" + " Likely reason: some sequences of plug and unplug" + " can break migration for machine versions prior" + " 2.7 (known design flaw).\n"); + error_report_err(err); + return -EINVAL; + } /* Re-assign subch. */ if (old_schid != s->schid) { old_s = channel_subsys.css[s->cssid]->sch_set[s->ssid]->sch[old_schid]; diff --git a/hw/s390x/virtio-ccw.c b/hw/s390x/virtio-ccw.c index e7167e3..4f7efa2 100644 --- a/hw/s390x/virtio-ccw.c +++ b/hw/s390x/virtio-ccw.c @@ -1274,9 +1274,13 @@ static int virtio_ccw_load_config(DeviceState *d, QEMUFile *f) SubchDev *s = ccw_dev->sch; VirtIODevice *vdev = virtio_ccw_get_vdev(s); int len; + int ret; s->driver_data = dev; - subch_device_load(s, f); + ret = subch_device_load(s, f); + if (ret) { + return ret; + } /* Re-fill subch_id after loading the subchannel states.*/ if (ck->refill_ids) { ck->refill_ids(ccw_dev); -- 2.10.2