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Re: [PATCH v3 3/3] migration/doc: We broke backwards compatibility
From: |
Michael S. Tsirkin |
Subject: |
Re: [PATCH v3 3/3] migration/doc: We broke backwards compatibility |
Date: |
Wed, 17 May 2023 06:20:55 -0400 |
On Mon, May 15, 2023 at 10:32:01AM +0200, Juan Quintela wrote:
> When we detect that we have broken backwards compantibility 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>
> ---
> docs/devel/migration.rst | 194 +++++++++++++++++++++++++++++++++++++++
> 1 file changed, 194 insertions(+)
>
> diff --git a/docs/devel/migration.rst b/docs/devel/migration.rst
> index 95e797ee60..97b6f48474 100644
> --- a/docs/devel/migration.rst
> +++ b/docs/devel/migration.rst
> @@ -451,6 +451,200 @@ 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 continously 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 9a6ef182c03eaa138bae553f0fbb5a123bef9a53
> + Author: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> + Date: Thu Mar 2 13:37:03 2023 +0000
> +
> + hw/pci/aer: Add missing routing for AER errors
> +
> +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 betwwen source and
> +destination.
> +
> +The following commit show how this got fixed:
> +
> +<put info of the commit once that it arrives upstream>
> +
> +The relevant parts of the fix 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 te 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 disable 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 restaured 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 relased 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 gonig to do this migration:
> +
> +- $ qemu-8.0 -M pc-7.2 -> qemu-8.0.1 -M pc-7.2
> +
> +We can launche the appropiate 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 mapchine to poweroff/poweron on a fixed qemu. And from now
> +on we can use the proper real machine.
> +
> VMState
> -------
Can we release this list of things that need to be configured
somewhere? Maybe in a sane format that libvirt can parse?
> --
> 2.40.1