[PATCH v2 0/3] Migration Documentation for backward compatibility

[PATCH v2 0/3] Migration Documentation for backward compatibility

[Juan Quintela]

Hi

This series contains the documentation for migration.  It is intentded
for both qemu developers and developers of management applications.

Please, review.

Juan Quintela (3):
  migration/doc: Add documentation for backwards compatiblity
  migration/doc: How to migrate when hosts have different features
  migration/doc: We broke backwards compatibility

 docs/devel/migration.rst | 529 ++++++++++++++++++++++++++++++++++++++-
 1 file changed, 522 insertions(+), 7 deletions(-)

-- 
2.41.0

[PATCH v2 1/3] migration/doc: Add documentation for backwards compatiblity

[Juan Quintela]

State what are the requeriments to get migration working between qemu
versions.  And once there explain how one is supposed to implement a
new feature/default value and not break migration.

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Acked-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
---
 docs/devel/migration.rst | 217 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 217 insertions(+)

diff --git a/docs/devel/migration.rst b/docs/devel/migration.rst
index c3e1400c0c..5ef2b36e9e 100644
--- a/docs/devel/migration.rst
+++ b/docs/devel/migration.rst
@@ -142,6 +142,223 @@ General advice for device developers
   may be different on the destination.  This can result in the
   device state being loaded into the wrong device.
 
+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"},
+         ...
+     };
+
+
 VMState
 -------
 
-- 
2.41.0

Re: [PATCH v2 1/3] migration/doc: Add documentation for backwards compatiblity

[Peter Maydell]

On Tue, 17 Oct 2023 at 16:20, Juan Quintela <quintela@redhat.com> wrote:
>
> State what are the requeriments to get migration working between qemu
> versions.  And once there explain how one is supposed to implement a
> new feature/default value and not break migration.
>
> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> Acked-by: Peter Xu <peterx@redhat.com>
> Signed-off-by: Juan Quintela <quintela@redhat.com>
> ---
>  docs/devel/migration.rst | 217 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 217 insertions(+)
>
> diff --git a/docs/devel/migration.rst b/docs/devel/migration.rst
> index c3e1400c0c..5ef2b36e9e 100644
> --- a/docs/devel/migration.rst
> +++ b/docs/devel/migration.rst
> @@ -142,6 +142,223 @@ General advice for device developers
>    may be different on the destination.  This can result in the
>    device state being loaded into the wrong device.
>
> +How backwards compatibility works
> +---------------------------------

Is there a better place in this document that we could add
this information? At the moment these new sections have
been placed in the middle of the "Saving the state of one device"
section, which doesn't seem right; worse, it's between the
"General advice for device developers" and the "VMState"
subsections of that section, which means that we end up with
a very long part of the document that is talking about edge
cases, and which splits up the two parts of the document that
are really all most device model authors need to think about.

My guess is that the extra documentation on backwards
compatibility that these patches are adding should get
a new section of its own (at the same level as the current
top-level sections "Transports", "Debugging", etc) which
then has its own subsections; and that this should probably
go either at the bottom of the document, after "Firmware",
or else just before "Stream structure".

We should probably also add a
.. contents::
directive near the start of the page, since it's now big
enough and has enough sections and subsections that a
table of contents would be helpful.

thanks
-- PMM

Re: [PATCH v2 1/3] migration/doc: Add documentation for backwards compatiblity

[Juan Quintela]

Peter Maydell <peter.maydell@linaro.org> wrote:
> On Tue, 17 Oct 2023 at 16:20, Juan Quintela <quintela@redhat.com> wrote:
>>
>> State what are the requeriments to get migration working between qemu
>> versions.  And once there explain how one is supposed to implement a
>> new feature/default value and not break migration.
>>
>> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
>> Acked-by: Peter Xu <peterx@redhat.com>
>> Signed-off-by: Juan Quintela <quintela@redhat.com>
>> ---
>>  docs/devel/migration.rst | 217 +++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 217 insertions(+)
>>
>> diff --git a/docs/devel/migration.rst b/docs/devel/migration.rst
>> index c3e1400c0c..5ef2b36e9e 100644
>> --- a/docs/devel/migration.rst
>> +++ b/docs/devel/migration.rst
>> @@ -142,6 +142,223 @@ General advice for device developers
>>    may be different on the destination.  This can result in the
>>    device state being loaded into the wrong device.
>>
>> +How backwards compatibility works
>> +---------------------------------
>
> Is there a better place in this document that we could add
> this information?

> At the moment these new sections have
> been placed in the middle of the "Saving the state of one device"
> section, which doesn't seem right; worse, it's between the
> "General advice for device developers" and the "VMState"
> subsections of that section, which means that we end up with
> a very long part of the document that is talking about edge
> cases, and which splits up the two parts of the document that
> are really all most device model authors need to think about.

You are right.  I didn't know for a good place either.
I created a new section:

Backwards compatibility

At the end of the archive.

> My guess is that the extra documentation on backwards
> compatibility that these patches are adding should get
> a new section of its own (at the same level as the current
> top-level sections "Transports", "Debugging", etc) which
> then has its own subsections; and that this should probably
> go either at the bottom of the document, after "Firmware",
> or else just before "Stream structure".

End it is.

> We should probably also add a
> .. contents::
> directive near the start of the page, since it's now big
> enough and has enough sections and subsections that a
> table of contents would be helpful.

Also done.

Thanks a lot.

[PATCH v2 2/3] migration/doc: 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>
Acked-by: Peter Xu <peterx@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 | 96 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 96 insertions(+)

diff --git a/docs/devel/migration.rst b/docs/devel/migration.rst
index 5ef2b36e9e..e671df729e 100644
--- a/docs/devel/migration.rst
+++ b/docs/devel/migration.rst
@@ -358,6 +358,102 @@ 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.
+
+Notice that we don't recomend to use -cpu host for migration.  It is
+used in this example because it makes the exampler 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.
 
 VMState
 -------
-- 
2.41.0

Re: [PATCH v2 2/3] migration/doc: How to migrate when hosts have different features

[Fabiano Rosas]

Juan Quintela <quintela@redhat.com> writes:

> 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>
> Acked-by: Peter Xu <peterx@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 | 96 ++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 96 insertions(+)
>
> diff --git a/docs/devel/migration.rst b/docs/devel/migration.rst
> index 5ef2b36e9e..e671df729e 100644
> --- a/docs/devel/migration.rst
> +++ b/docs/devel/migration.rst
> @@ -358,6 +358,102 @@ 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

s/handle/handled/

> +application that launches QEMU.  If QEMU is configured correctly, the
> +migration will suceeed.

s/suceeed/succeed/

> +
> +Once that we have defined that, doing this is complicated.  Almost all

I get what you mean here, but it is slightly confusing. Maybe

"Once we have defined that, doing it is complicated." or
"That said, actually doing it is complicated."

> +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.

s/waranteed/guaranteed/

> +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

s/don't/doesn't/

> +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 exampler simpler.

s/exampler/example/

> +
> +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
>  -------

Re: [PATCH v2 2/3] migration/doc: How to migrate when hosts have different features

[Juan Quintela]

Fabiano Rosas <farosas@suse.de> wrote:
> Juan Quintela <quintela@redhat.com> writes:
>> +Notice that this is not completely related to QEMU.  The most
>> +important thing here is that this should be handle by the managing
>
> s/handle/handled/

Done.

>> +application that launches QEMU.  If QEMU is configured correctly, the
>> +migration will suceeed.
>
> s/suceeed/succeed/

Done.

>
>> +
>> +Once that we have defined that, doing this is complicated.  Almost all
>
> I get what you mean here, but it is slightly confusing. Maybe
>
> "Once we have defined that, doing it is complicated." or
> "That said, actually doing it is complicated."

Last one wins.

>> +When both hosts have different cpu features this is waranteed to
>> fail.
>
> s/waranteed/guaranteed/

Done.
>> +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
>
> s/don't/doesn't/

Done

>> +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 exampler simpler.
>
> s/exampler/example/

Thanks.

[PATCH v2 3/3] migration/doc: We broke backwards compatibility

[Juan Quintela]

When we detect that we have broken backwards compatibility in a
released version, we can't do anything for that version.  But once we
fix that bug on the next released version, we can "mitigate" that
problem when migrating to new versions to give a way out of that
machine until it does a hard reboot.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
---
 docs/devel/migration.rst | 216 +++++++++++++++++++++++++++++++++++++--
 1 file changed, 209 insertions(+), 7 deletions(-)

diff --git a/docs/devel/migration.rst b/docs/devel/migration.rst
index e671df729e..6146f95a8b 100644
--- a/docs/devel/migration.rst
+++ b/docs/devel/migration.rst
@@ -455,11 +455,213 @@ 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.
 
+How to mitigate when we have a backward compatibility error
+-----------------------------------------------------------
+
+We broke migration for old machine types continuously during
+development.  But as soon as we find that there is a problem, we fix
+it.  The problem is what happens when we detect after we have done a
+release that something has gone wrong.
+
+Let see how it worked with one example.
+
+After the release of qemu-8.0 we found a problem when doing migration
+of the machine type pc-7.2.
+
+- $ qemu-7.2 -M pc-7.2  ->  qemu-7.2 -M pc-7.2
+
+  This migration works
+
+- $ qemu-8.0 -M pc-7.2  ->  qemu-8.0 -M pc-7.2
+
+  This migration works
+
+- $ qemu-8.0 -M pc-7.2  ->  qemu-7.2 -M pc-7.2
+
+  This migration fails
+
+- $ qemu-7.2 -M pc-7.2  ->  qemu-8.0 -M pc-7.2
+
+  This migration fails
+
+So clearly something fails when migration between qemu-7.2 and
+qemu-8.0 with machine type pc-7.2.  The error messages, and git bisect
+pointed to this commit.
+
+In qemu-8.0 we got this commit: ::
+
+    commit 010746ae1db7f52700cb2e2c46eb94f299cfa0d2
+    Author: Jonathan Cameron <Jonathan.Cameron@huawei.com>
+    Date:   Thu Mar 2 13:37:02 2023 +0000
+
+    hw/pci/aer: Implement PCI_ERR_UNCOR_MASK register
+
+
+The relevant bits of the commit for our example are this ones:
+
+    --- a/hw/pci/pcie_aer.c
+    +++ b/hw/pci/pcie_aer.c
+    @@ -112,6 +112,10 @@ int pcie_aer_init(PCIDevice *dev,
+
+         pci_set_long(dev->w1cmask + offset + PCI_ERR_UNCOR_STATUS,
+                      PCI_ERR_UNC_SUPPORTED);
+    +    pci_set_long(dev->config + offset + PCI_ERR_UNCOR_MASK,
+    +                 PCI_ERR_UNC_MASK_DEFAULT);
+    +    pci_set_long(dev->wmask + offset + PCI_ERR_UNCOR_MASK,
+    +                 PCI_ERR_UNC_SUPPORTED);
+
+         pci_set_long(dev->config + offset + PCI_ERR_UNCOR_SEVER,
+                     PCI_ERR_UNC_SEVERITY_DEFAULT);
+
+The patch changes how we configure PCI space for AER.  But QEMU fails
+when the PCI space configuration is different between source and
+destination.
+
+The following commit shows how this got fixed:
+
+    commit 5ed3dabe57dd9f4c007404345e5f5bf0e347317f
+    Author: Leonardo Bras <leobras@redhat.com>
+    Date:   Tue May 2 21:27:02 2023 -0300
+
+    hw/pci: Disable PCI_ERR_UNCOR_MASK register for machine type < 8.0
+
+    [...]
+
+The relevant parts of the fix in QEMU are as follow:
+
+First, we create a new property for the device to be able to configure
+the old behaviour or the new behaviour. ::
+
+    diff --git a/hw/pci/pci.c b/hw/pci/pci.c
+    index 8a87ccc8b0..5153ad63d6 100644
+    --- a/hw/pci/pci.c
+    +++ b/hw/pci/pci.c
+    @@ -79,6 +79,8 @@ static Property pci_props[] = {
+         DEFINE_PROP_STRING("failover_pair_id", PCIDevice,
+                            failover_pair_id),
+         DEFINE_PROP_UINT32("acpi-index",  PCIDevice, acpi_index, 0),
+    +    DEFINE_PROP_BIT("x-pcie-err-unc-mask", PCIDevice, cap_present,
+    +                    QEMU_PCIE_ERR_UNC_MASK_BITNR, true),
+         DEFINE_PROP_END_OF_LIST()
+     };
+
+Notice that we enable the feature for new machine types.
+
+Now we see how the fix is done.  This is going to depend on what kind
+of breakage happens, but in this case it is quite simple. ::
+
+    diff --git a/hw/pci/pcie_aer.c b/hw/pci/pcie_aer.c
+    index 103667c368..374d593ead 100644
+    --- a/hw/pci/pcie_aer.c
+    +++ b/hw/pci/pcie_aer.c
+    @@ -112,10 +112,13 @@ int pcie_aer_init(PCIDevice *dev, uint8_t cap_ver,
+    uint16_t offset,
+
+         pci_set_long(dev->w1cmask + offset + PCI_ERR_UNCOR_STATUS,
+                      PCI_ERR_UNC_SUPPORTED);
+    -    pci_set_long(dev->config + offset + PCI_ERR_UNCOR_MASK,
+    -                 PCI_ERR_UNC_MASK_DEFAULT);
+    -    pci_set_long(dev->wmask + offset + PCI_ERR_UNCOR_MASK,
+    -                 PCI_ERR_UNC_SUPPORTED);
+    +
+    +    if (dev->cap_present & QEMU_PCIE_ERR_UNC_MASK) {
+    +        pci_set_long(dev->config + offset + PCI_ERR_UNCOR_MASK,
+    +                     PCI_ERR_UNC_MASK_DEFAULT);
+    +        pci_set_long(dev->wmask + offset + PCI_ERR_UNCOR_MASK,
+    +                     PCI_ERR_UNC_SUPPORTED);
+    +    }
+
+         pci_set_long(dev->config + offset + PCI_ERR_UNCOR_SEVER,
+                      PCI_ERR_UNC_SEVERITY_DEFAULT);
+
+I.e. If the property bit is enabled, we configure it as we did for
+qemu-8.0.  If the property bit is not set, we configure it as it was in 7.2.
+
+And now, everything that is missing is disabling the feature for old
+machine types: ::
+
+    diff --git a/hw/core/machine.c b/hw/core/machine.c
+    index 47a34841a5..07f763eb2e 100644
+    --- a/hw/core/machine.c
+    +++ b/hw/core/machine.c
+    @@ -48,6 +48,7 @@ GlobalProperty hw_compat_7_2[] = {
+         { "e1000e", "migrate-timadj", "off" },
+         { "virtio-mem", "x-early-migration", "false" },
+         { "migration", "x-preempt-pre-7-2", "true" },
+    +    { TYPE_PCI_DEVICE, "x-pcie-err-unc-mask", "off" },
+     };
+     const size_t hw_compat_7_2_len = G_N_ELEMENTS(hw_compat_7_2);
+
+And now, when qemu-8.0.1 is released with this fix, all combinations
+are going to work as supposed.
+
+- $ qemu-7.2 -M pc-7.2  ->  qemu-7.2 -M pc-7.2 (works)
+- $ qemu-8.0.1 -M pc-7.2  ->  qemu-8.0.1 -M pc-7.2 (works)
+- $ qemu-8.0.1 -M pc-7.2  ->  qemu-7.2 -M pc-7.2 (works)
+- $ qemu-7.2 -M pc-7.2  ->  qemu-8.0.1 -M pc-7.2 (works)
+
+So the normality has been restored and everything is ok, no?
+
+Not really, now our matrix is much bigger.  We started with the easy
+cases, migration from the same version to the same version always
+works:
+
+- $ qemu-7.2 -M pc-7.2  ->  qemu-7.2 -M pc-7.2
+- $ qemu-8.0 -M pc-7.2  ->  qemu-8.0 -M pc-7.2
+- $ qemu-8.0.1 -M pc-7.2  ->  qemu-8.0.1 -M pc-7.2
+
+Now the interesting ones.  When the QEMU processes versions are
+different.  For the 1st set, their fail and we can do nothing, both
+versions are released and we can't change anything.
+
+- $ qemu-7.2 -M pc-7.2  ->  qemu-8.0 -M pc-7.2
+- $ qemu-8.0 -M pc-7.2  ->  qemu-7.2 -M pc-7.2
+
+This two are the ones that work. The whole point of making the
+change in qemu-8.0.1 release was to fix this issue:
+
+- $ qemu-7.2 -M pc-7.2  ->  qemu-8.0.1 -M pc-7.2
+- $ qemu-8.0.1 -M pc-7.2  ->  qemu-7.2 -M pc-7.2
+
+But now we found that qemu-8.0 neither can migrate to qemu-7.2 not
+qemu-8.0.1.
+
+- $ qemu-8.0 -M pc-7.2  ->  qemu-8.0.1 -M pc-7.2
+- $ qemu-8.0.1 -M pc-7.2  ->  qemu-8.0 -M pc-7.2
+
+So, if we start a pc-7.2 machine in qemu-8.0 we can't migrate it to
+anything except to qemu-8.0.
+
+Can we do better?
+
+Yeap.  If we know that we are going to do this migration:
+
+- $ qemu-8.0 -M pc-7.2  ->  qemu-8.0.1 -M pc-7.2
+
+We can launch the appropriate devices with
+
+--device...,x-pci-e-err-unc-mask=on
+
+And now we can receive a migration from 8.0.  And from now on, we can
+do that migration to new machine types if we remember to enable that
+property for pc-7.2.  Notice that we need to remember, it is not
+enough to know that the source of the migration is qemu-8.0.  Think of
+this example:
+
+$ qemu-8.0 -M pc-7.2 -> qemu-8.0.1 -M pc-7.2 -> qemu-8.2 -M pc-7.2
+
+In the second migration, the source is not qemu-8.0, but we still have
+that "problem" and have that property enabled.  Notice that we need to
+continue having this mark/property until we have this machine
+rebooted.  But it is not a normal reboot (that don't reload QEMU) we
+need the machine to poweroff/poweron on a fixed QEMU.  And from now
+on we can use the proper real machine.
+
 VMState
 -------
 
-Most device data can be described using the ``VMSTATE`` macros (mostly defined
-in ``include/migration/vmstate.h``).
+Most device data can be described using the ``VMSTATE`` macros (mostly
+defined in ``include/migration/vmstate.h``).
 
 An example (from hw/input/pckbd.c)
 
@@ -478,16 +680,16 @@ An example (from hw/input/pckbd.c)
       }
   };
 
-We are declaring the state with name "pckbd".
-The ``version_id`` is 3, and the fields are 4 uint8_t in a KBDState structure.
-We registered this with:
+We are declaring the state with name "pckbd".  The ``version_id`` is
+3, and the fields are 4 uint8_t in a KBDState structure.  We
+registered this with:
 
 .. code:: c
 
     vmstate_register(NULL, 0, &vmstate_kbd, s);
 
-For devices that are ``qdev`` based, we can register the device in the class
-init function:
+For devices that are ``qdev`` based, we can register the device in the
+class init function:
 
 .. code:: c
 
-- 
2.41.0

Re: [PATCH v2 3/3] migration/doc: We broke backwards compatibility

[Fabiano Rosas]

Juan Quintela <quintela@redhat.com> writes:

> When we detect that we have broken backwards compatibility in a
> released version, we can't do anything for that version.  But once we
> fix that bug on the next released version, we can "mitigate" that
> problem when migrating to new versions to give a way out of that
> machine until it does a hard reboot.
>
> Signed-off-by: Juan Quintela <quintela@redhat.com>
> Acked-by: Peter Xu <peterx@redhat.com>

Reviewed-by: Fabiano Rosas <farosas@suse.de>