Sometimes devices have different features depending of things outside
of qemu. For instance the kernel. Document how to handle that cases.
Acked-by: Peter Xu <peterx@xxxxxxxxxx>
Reviewed-by: Fabiano Rosas <farosas@xxxxxxx>
Signed-off-by: Juan Quintela <quintela@xxxxxxxxxx>
Message-ID: <20231018112827.1325-4-quintela@xxxxxxxxxx>
---
docs/devel/migration.rst | 97 ++++++++++++++++++++++++++++++++++++++++
1 file changed, 97 insertions(+)
diff --git a/docs/devel/migration.rst b/docs/devel/migration.rst
index 6fe275b1ec..974505e4a7 100644
--- a/docs/devel/migration.rst
+++ b/docs/devel/migration.rst
@@ -1138,3 +1138,100 @@ machine types to have the right value::
+ { "virtio-blk-device", "num-queues", "1"},
...
};
+
+A device with diferent features on both sides
+---------------------------------------------
+
+Let's assume that we are using the same QEMU binary on both sides,
+just to make the things easier. But we have a device that has
+different features on both sides of the migration. That can be
+because the devices are different, because the kernel driver of both
+devices have different features, whatever.
+
+How can we get this to work with migration. The way to do that is
+"theoretically" easy. You have to get the features that the device
+has in the source of the migration. The features that the device has
+on the target of the migration, you get the intersection of the
+features of both sides, and that is the way that you should launch
+QEMU.
+
+Notice that this is not completely related to QEMU. The most
+important thing here is that this should be handled by the managing
+application that launches QEMU. If QEMU is configured correctly, the
+migration will succeed.
+
+That said, actually doing it is complicated. Almost all devices are
+bad at being able to be launched with only some features enabled.
+With one big exception: cpus.
+
+You can read the documentation for QEMU x86 cpu models here:
+
+https://qemu-project.gitlab.io/qemu/system/qemu-cpu-models.html
+
+See when they talk about migration they recommend that one chooses the
+newest cpu model that is supported for all cpus.
+
+Let's say that we have:
+
+Host A:
+
+Device X has the feature Y
+
+Host B:
+
+Device X has not the feature Y
+
+If we try to migrate without any care from host A to host B, it will
+fail because when migration tries to load the feature Y on
+destination, it will find that the hardware is not there.
+
+Doing this would be the equivalent of doing with cpus:
+
+Host A:
+
+$ qemu-system-x86_64 -cpu host
+
+Host B:
+
+$ qemu-system-x86_64 -cpu host
+
+When both hosts have different cpu features this is guaranteed to
+fail. Especially if Host B has less features than host A. If host A
+has less features than host B, sometimes it works. Important word of
+last sentence is "sometimes".
+
+So, forgetting about cpu models and continuing with the -cpu host
+example, let's see that the differences of the cpus is that Host A and
+B have the following features:
+
+Features: 'pcid' 'stibp' 'taa-no'
+Host A: X X
+Host B: X
+
+And we want to migrate between them, the way configure both QEMU cpu
+will be:
+
+Host A:
+
+$ qemu-system-x86_64 -cpu host,pcid=off,stibp=off
+
+Host B:
+
+$ qemu-system-x86_64 -cpu host,taa-no=off
+
+And you would be able to migrate between them. It is responsability
+of the management application or of the user to make sure that the
+configuration is correct. QEMU doesn't know how to look at this kind
+of features in general.
+
+Notice that we don't recomend to use -cpu host for migration. It is
+used in this example because it makes the example simpler.
+
+Other devices have worse control about individual features. If they
+want to be able to migrate between hosts that show different features,
+the device needs a way to configure which ones it is going to use.
+
+In this section we have considered that we are using the same QEMU
+binary in both sides of the migration. If we use different QEMU
+versions process, then we need to have into account all other
+differences and the examples become even more complicated.
--
2.41.0
