@@ -919,3 +919,222 @@ versioned machine types to cut down on the combinations that will need
support. This is also useful when newer versions of firmware outgrow
the padding.
+
+Backwards compatibility
+=======================
+
+How backwards compatibility works
+---------------------------------
+
+When we do migration, we have two QEMU processes: the source and the
+target. There are two cases, they are the same version or they are
+different versions. The easy case is when they are the same version.
+The difficult one is when they are different versions.
+
+There are two things that are different, but they have very similar
+names and sometimes get confused:
+
+- QEMU version
+- machine type version
+
+Let's start with a practical example, we start with:
+
+- qemu-system-x86_64 (v5.2), from now on qemu-5.2.
+- qemu-system-x86_64 (v5.1), from now on qemu-5.1.
+
+Related to this are the "latest" machine types defined on each of
+them:
+
+- pc-q35-5.2 (newer one in qemu-5.2) from now on pc-5.2
+- pc-q35-5.1 (newer one in qemu-5.1) from now on pc-5.1
+
+First of all, migration is only supposed to work if you use the same
+machine type in both source and destination. The QEMU hardware
+configuration needs to be the same also on source and destination.
+Most aspects of the backend configuration can be changed at will,
+except for a few cases where the backend features influence frontend
+device feature exposure. But that is not relevant for this section.
+
+I am going to list the number of combinations that we can have. Let's
+start with the trivial ones, QEMU is the same on source and
+destination:
+
+1 - qemu-5.2 -M pc-5.2 -> migrates to -> qemu-5.2 -M pc-5.2
+
+ This is the latest QEMU with the latest machine type.
+ This have to work, and if it doesn't work it is a bug.
+
+2 - qemu-5.1 -M pc-5.1 -> migrates to -> qemu-5.1 -M pc-5.1
+
+ Exactly the same case than the previous one, but for 5.1.
+ Nothing to see here either.
+
+This are the easiest ones, we will not talk more about them in this
+section.
+
+Now we start with the more interesting cases. Consider the case where
+we have the same QEMU version in both sides (qemu-5.2) but we are using
+the latest machine type for that version (pc-5.2) but one of an older
+QEMU version, in this case pc-5.1.
+
+3 - qemu-5.2 -M pc-5.1 -> migrates to -> qemu-5.2 -M pc-5.1
+
+ It needs to use the definition of pc-5.1 and the devices as they
+ were configured on 5.1, but this should be easy in the sense that
+ both sides are the same QEMU and both sides have exactly the same
+ idea of what the pc-5.1 machine is.
+
+4 - qemu-5.1 -M pc-5.2 -> migrates to -> qemu-5.1 -M pc-5.2
+
+ This combination is not possible as the qemu-5.1 doen't understand
+ pc-5.2 machine type. So nothing to worry here.
+
+Now it comes the interesting ones, when both QEMU processes are
+different. Notice also that the machine type needs to be pc-5.1,
+because we have the limitation than qemu-5.1 doesn't know pc-5.2. So
+the possible cases are:
+
+5 - qemu-5.2 -M pc-5.1 -> migrates to -> qemu-5.1 -M pc-5.1
+
+ This migration is known as newer to older. We need to make sure
+ when we are developing 5.2 we need to take care about not to break
+ migration to qemu-5.1. Notice that we can't make updates to
+ qemu-5.1 to understand whatever qemu-5.2 decides to change, so it is
+ in qemu-5.2 side to make the relevant changes.
+
+6 - qemu-5.1 -M pc-5.1 -> migrates to -> qemu-5.2 -M pc-5.1
+
+ This migration is known as older to newer. We need to make sure
+ than we are able to receive migrations from qemu-5.1. The problem is
+ similar to the previous one.
+
+If qemu-5.1 and qemu-5.2 were the same, there will not be any
+compatibility problems. But the reason that we create qemu-5.2 is to
+get new features, devices, defaults, etc.
+
+If we get a device that has a new feature, or change a default value,
+we have a problem when we try to migrate between different QEMU
+versions.
+
+So we need a way to tell qemu-5.2 that when we are using machine type
+pc-5.1, it needs to **not** use the feature, to be able to migrate to
+real qemu-5.1.
+
+And the equivalent part when migrating from qemu-5.1 to qemu-5.2.
+qemu-5.2 has to expect that it is not going to get data for the new
+feature, because qemu-5.1 doesn't know about it.
+
+How do we tell QEMU about these device feature changes? In
+hw/core/machine.c:hw_compat_X_Y arrays.
+
+If we change a default value, we need to put back the old value on
+that array. And the device, during initialization needs to look at
+that array to see what value it needs to get for that feature. And
+what are we going to put in that array, the value of a property.
+
+To create a property for a device, we need to use one of the
+DEFINE_PROP_*() macros. See include/hw/qdev-properties.h to find the
+macros that exist. With it, we set the default value for that
+property, and that is what it is going to get in the latest released
+version. But if we want a different value for a previous version, we
+can change that in the hw_compat_X_Y arrays.
+
+hw_compat_X_Y is an array of registers that have the format:
+
+- name_device
+- name_property
+- value
+
+Let's see a practical example.
+
+In qemu-5.2 virtio-blk-device got multi queue support. This is a
+change that is not backward compatible. In qemu-5.1 it has one
+queue. In qemu-5.2 it has the same number of queues as the number of
+cpus in the system.
+
+When we are doing migration, if we migrate from a device that has 4
+queues to a device that have only one queue, we don't know where to
+put the extra information for the other 3 queues, and we fail
+migration.
+
+Similar problem when we migrate from qemu-5.1 that has only one queue
+to qemu-5.2, we only sent information for one queue, but destination
+has 4, and we have 3 queues that are not properly initialized and
+anything can happen.
+
+So, how can we address this problem. Easy, just convince qemu-5.2
+that when it is running pc-5.1, it needs to set the number of queues
+for virtio-blk-devices to 1.
+
+That way we fix the cases 5 and 6.
+
+5 - qemu-5.2 -M pc-5.1 -> migrates to -> qemu-5.1 -M pc-5.1
+
+ qemu-5.2 -M pc-5.1 sets number of queues to be 1.
+ qemu-5.1 -M pc-5.1 expects number of queues to be 1.
+
+ correct. migration works.
+
+6 - qemu-5.1 -M pc-5.1 -> migrates to -> qemu-5.2 -M pc-5.1
+
+ qemu-5.1 -M pc-5.1 sets number of queues to be 1.
+ qemu-5.2 -M pc-5.1 expects number of queues to be 1.
+
+ correct. migration works.
+
+And now the other interesting case, case 3. In this case we have:
+
+3 - qemu-5.2 -M pc-5.1 -> migrates to -> qemu-5.2 -M pc-5.1
+
+ Here we have the same QEMU in both sides. So it doesn't matter a
+ lot if we have set the number of queues to 1 or not, because
+ they are the same.
+
+ WRONG!
+
+ Think what happens if we do one of this double migrations:
+
+ A -> migrates -> B -> migrates -> C
+
+ where:
+
+ A: qemu-5.1 -M pc-5.1
+ B: qemu-5.2 -M pc-5.1
+ C: qemu-5.2 -M pc-5.1
+
+ migration A -> B is case 6, so number of queues needs to be 1.
+
+ migration B -> C is case 3, so we don't care. But actually we
+ care because we haven't started the guest in qemu-5.2, it came
+ migrated from qemu-5.1. So to be in the safe place, we need to
+ always use number of queues 1 when we are using pc-5.1.
+
+Now, how was this done in reality? The following commit shows how it
+was done::
+
+ commit 9445e1e15e66c19e42bea942ba810db28052cd05
+ Author: Stefan Hajnoczi <stefanha@redhat.com>
+ Date: Tue Aug 18 15:33:47 2020 +0100
+
+ virtio-blk-pci: default num_queues to -smp N
+
+The relevant parts for migration are::
+
+ @@ -1281,7 +1284,8 @@ static Property virtio_blk_properties[] = {
+ #endif
+ DEFINE_PROP_BIT("request-merging", VirtIOBlock, conf.request_merging, 0,
+ true),
+ - DEFINE_PROP_UINT16("num-queues", VirtIOBlock, conf.num_queues, 1),
+ + DEFINE_PROP_UINT16("num-queues", VirtIOBlock, conf.num_queues,
+ + VIRTIO_BLK_AUTO_NUM_QUEUES),
+ DEFINE_PROP_UINT16("queue-size", VirtIOBlock, conf.queue_size, 256),
+
+It changes the default value of num_queues. But it fishes it for old
+machine types to have the right value::
+
+ @@ -31,6 +31,7 @@
+ GlobalProperty hw_compat_5_1[] = {
+ ...
+ + { "virtio-blk-device", "num-queues", "1"},
+ ...
+ };