"Michael S. Tsirkin" <m...@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 <quint...@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.came...@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? 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. Daniel, Jiri, what would you need and what would be useful to you? Later, Juan.