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diff --git a/docs/devel/migration.rst b/docs/devel/migration.rst index 4d6a98ae58..6fe275b1ec 100644 --- a/docs/devel/migration.rst +++ b/docs/devel/migration.rst @@ -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"}, + ... + }; |