devel/migration/compatibility.rst

20 - qemu-system-x86_64 (v5.2), from now on qemu-5.2.
21 - qemu-system-x86_64 (v5.1), from now on qemu-5.1.
26 - pc-q35-5.2 (newer one in qemu-5.2) from now on pc-5.2
27 - pc-q35-5.1 (newer one in qemu-5.1) from now on pc-5.1
40 1 - qemu-5.2 -M pc-5.2  -> migrates to -> qemu-5.2 -M pc-5.2
45 2 - qemu-5.1 -M pc-5.1  -> migrates to -> qemu-5.1 -M pc-5.1
54 we have the same QEMU version in both sides (qemu-5.2) but we are using
58 3 - qemu-5.2 -M pc-5.1  -> migrates to -> qemu-5.2 -M pc-5.1
65 4 - qemu-5.1 -M pc-5.2  -> migrates to -> qemu-5.1 -M pc-5.2
67   This combination is not possible as the qemu-5.1 doesn't understand
72 because we have the limitation than qemu-5.1 doesn't know pc-5.2.  So
75 5 - qemu-5.2 -M pc-5.1  -> migrates to -> qemu-5.1 -M pc-5.1
79   migration to qemu-5.1.  Notice that we can't make updates to
80   qemu-5.1 to understand whatever qemu-5.2 decides to change, so it is
81   in qemu-5.2 side to make the relevant changes.
83 6 - qemu-5.1 -M pc-5.1  -> migrates to -> qemu-5.2 -M pc-5.1
86   than we are able to receive migrations from qemu-5.1. The problem is
89 If qemu-5.1 and qemu-5.2 were the same, there will not be any
90 compatibility problems.  But the reason that we create qemu-5.2 is to
97 So we need a way to tell qemu-5.2 that when we are using machine type
99 real qemu-5.1.
101 And the equivalent part when migrating from qemu-5.1 to qemu-5.2.
102 qemu-5.2 has to expect that it is not going to get data for the new
103 feature, because qemu-5.1 doesn't know about it.
128 In qemu-5.2 virtio-blk-device got multi queue support.  This is a
129 change that is not backward compatible.  In qemu-5.1 it has one
130 queue. In qemu-5.2 it has the same number of queues as the number of
138 Similar problem when we migrate from qemu-5.1 that has only one queue
139 to qemu-5.2, we only sent information for one queue, but destination
143 So, how can we address this problem.  Easy, just convince qemu-5.2
149 5 - qemu-5.2 -M pc-5.1  -> migrates to -> qemu-5.1 -M pc-5.1
151     qemu-5.2 -M pc-5.1 sets number of queues to be 1.
152     qemu-5.1 -M pc-5.1 expects number of queues to be 1.
156 6 - qemu-5.1 -M pc-5.1  -> migrates to -> qemu-5.2 -M pc-5.1
158     qemu-5.1 -M pc-5.1 sets number of queues to be 1.
159     qemu-5.2 -M pc-5.1 expects number of queues to be 1.
165 3 - qemu-5.2 -M pc-5.1  -> migrates to -> qemu-5.2 -M pc-5.1
179     A: qemu-5.1 -M pc-5.1
180     B: qemu-5.2 -M pc-5.1
181     C: qemu-5.2 -M pc-5.1
186     care because we haven't started the guest in qemu-5.2, it came
187     migrated from qemu-5.1.  So to be in the safe place, we need to
247 https://qemu-project.gitlab.io/qemu/system/qemu-cpu-models.html
270 $ qemu-system-x86_64 -cpu host
274 $ qemu-system-x86_64 -cpu host
294 $ qemu-system-x86_64 -cpu host,pcid=off,stibp=off
298 $ qemu-system-x86_64 -cpu host,taa-no=off
327 After the release of qemu-8.0 we found a problem when doing migration
330 - $ qemu-7.2 -M pc-7.2  ->  qemu-7.2 -M pc-7.2
334 - $ qemu-8.0 -M pc-7.2  ->  qemu-8.0 -M pc-7.2
338 - $ qemu-8.0 -M pc-7.2  ->  qemu-7.2 -M pc-7.2
342 - $ qemu-7.2 -M pc-7.2  ->  qemu-8.0 -M pc-7.2
346 So clearly something fails when migration between qemu-7.2 and
347 qemu-8.0 with machine type pc-7.2.  The error messages, and git bisect
350 In qemu-8.0 we got this commit::
437 qemu-8.0.  If the property bit is not set, we configure it as it was in 7.2.
454 And now, when qemu-8.0.1 is released with this fix, all combinations
457 - $ qemu-7.2 -M pc-7.2  ->  qemu-7.2 -M pc-7.2 (works)
458 - $ qemu-8.0.1 -M pc-7.2  ->  qemu-8.0.1 -M pc-7.2 (works)
459 - $ qemu-8.0.1 -M pc-7.2  ->  qemu-7.2 -M pc-7.2 (works)
460 - $ qemu-7.2 -M pc-7.2  ->  qemu-8.0.1 -M pc-7.2 (works)
468 - $ qemu-7.2 -M pc-7.2  ->  qemu-7.2 -M pc-7.2
469 - $ qemu-8.0 -M pc-7.2  ->  qemu-8.0 -M pc-7.2
470 - $ qemu-8.0.1 -M pc-7.2  ->  qemu-8.0.1 -M pc-7.2
476 - $ qemu-7.2 -M pc-7.2  ->  qemu-8.0 -M pc-7.2
477 - $ qemu-8.0 -M pc-7.2  ->  qemu-7.2 -M pc-7.2
480 change in qemu-8.0.1 release was to fix this issue:
482 - $ qemu-7.2 -M pc-7.2  ->  qemu-8.0.1 -M pc-7.2
483 - $ qemu-8.0.1 -M pc-7.2  ->  qemu-7.2 -M pc-7.2
485 But now we found that qemu-8.0 neither can migrate to qemu-7.2 not
486 qemu-8.0.1.
488 - $ qemu-8.0 -M pc-7.2  ->  qemu-8.0.1 -M pc-7.2
489 - $ qemu-8.0.1 -M pc-7.2  ->  qemu-8.0 -M pc-7.2
491 So, if we start a pc-7.2 machine in qemu-8.0 we can't migrate it to
492 anything except to qemu-8.0.
498 - $ qemu-8.0 -M pc-7.2  ->  qemu-8.0.1 -M pc-7.2
507 enough to know that the source of the migration is qemu-8.0.  Think of
510 $ qemu-8.0 -M pc-7.2 -> qemu-8.0.1 -M pc-7.2 -> qemu-8.2 -M pc-7.2
512 In the second migration, the source is not qemu-8.0, but we still have