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

[Michael S. Tsirkin]

On Wed, May 17, 2023 at 01:43:46PM +0200, Juan Quintela wrote:
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > 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?
> 
> What do you mean here?
> 
> the x-pci-e-err-unc-mask=on?

Yes.

> The most similar thing that we have is pc/machine.c:hw_compat_x_y.
> 
> But that also include the things where we have done the things right.

Exactly.

> Daniel, Jiri, what would you need and what would be useful to you?
> 
> Later, Juan.