[PATCH v3 2/3] migration/docs: How to migrate when hosts have different features

[Juan Quintela]

Sometimes devices have different features depending of things outside
of qemu.  For instance the kernel.  Document how to handle that cases.

Signed-off-by: Juan Quintela <quintela@redhat.com>

---

If you have some example to put here, I am all ears.  I guess that
virtio-* with some features that are on qemu but not on all kernel
would do the trick, but I am not a virtio guru myself.  Patches
welcome.
---
 docs/devel/migration.rst | 93 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 93 insertions(+)

diff --git a/docs/devel/migration.rst b/docs/devel/migration.rst
index b4c4f3ec35..95e797ee60 100644
--- a/docs/devel/migration.rst
+++ b/docs/devel/migration.rst
@@ -357,6 +357,99 @@ machine types to have the right value: ::
          ...
      };
 
+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 handle by the managing
+application that launches qemu.  If qemu is configured correctly, the
+migration will suceeed.
+
+Once that we have defined that, doing this 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 waranteed 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 don't know how to look at this kind of
+features in general.
+
+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.
 
 VMState
 -------
-- 
2.40.1