Re: [PATCH v3 3/3] migration/doc: We broke backwards compatibility

["Michael S. Tsirkin" <mst@redhat.com>]

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