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

[Fiona Ebner]

Am 15.05.23 um 10:32 schrieb Juan Quintela:
> When we detect that we have broken backwards compantibility in a

compatibility

(...)

> +
> +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);
> +

These changes are not part of commit
9a6ef182c0 ("hw/pci/aer: Add missing routing for AER errors")
but rather the one before it, namely
010746ae1d ("hw/pci/aer: Implement PCI_ERR_UNCOR_MASK register")

> +The patch changes how we configure pci space for AER.  But qemu fails

Should QEMU and PCI be capitalized in the text parts?

> +when the pci space configuration is different betwwen source and

between

> +destination.
> +
> +The following commit show how this got fixed:

shows

(...)

> +
> +So the normality has been restaured and everything is ok, no?

restored

> +
> +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.

released

> +
> +- $ 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:

going

> +
> +- $ qemu-8.0 -M pc-7.2  ->  qemu-8.0.1 -M pc-7.2
> +
> +We can launche the appropiate devices with

"launch" was already pointed out by Peter, but there's also "appropriate"

> +
> +--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

machine

Best Regards,
Fiona