Re: [PATCH 1/2] qemu: Fix migration with custom XML

On Tue, Apr 16, 2024 at 19:53:25 +0200, Jiri Denemark wrote: > Ages ago origCPU in domain private data was introduced to provide > backward compatibility when migrating to an old libvirt, which did not > support fetching updated CPU definition from QEMU. Thus origCPU will > contain the original CPU definition before such update. But only if the > update actually changed anything. Let's always fill origCPU with the > original definition when starting a domain so that we can rely on it > being always set, even if it matches the updated definition. > > This fixes migration or save operations with custom domain XML after > commit v10.1.0-88-g14d3517410, which expected origCPU to be always set > to the CPU definition from inactive XML to check features explicitly > requested by a user. > > https://issues.redhat.com/browse/RHEL-30622 > > Signed-off-by: Jiri Denemark <jdenemar(a)redhat.com&gt; > --- > src/qemu/qemu_domain.c | 38 +++++++++++++++++++++----------------- > src/qemu/qemu_process.c | 14 ++------------ > 2 files changed, 23 insertions(+), 29 deletions(-) > > diff --git a/src/qemu/qemu_domain.c b/src/qemu/qemu_domain.c > index 6b869797a8..ca50d435d2 100644 > --- a/src/qemu/qemu_domain.c > +++ b/src/qemu/qemu_domain.c > @@ -11013,15 +11014,25 @@ qemuDomainUpdateCPU(virDomainObj *vm, > > *origCPU = NULL; > > - if (!cpu || !vm->def->cpu || > - !virQEMUCapsGet(priv->qemuCaps, QEMU_CAPS_QUERY_CPU_MODEL_EXPANSION) || > - virCPUDefIsEqual(vm->def->cpu, cpu, false)) > + if (!vm->def->cpu) > return 0; > > - if (!(cpu = virCPUDefCopy(cpu))) > + if (!virQEMUCapsGet(priv->qemuCaps, QEMU_CAPS_QUERY_CPU_MODEL_EXPANSION)) > + return 0; > + > + /* nothing to do if only topology part of CPU def is used */ > + if (vm->def->cpu->mode == VIR_CPU_MODE_CUSTOM && !vm->def->cpu->model) > + return 0; > + > + if (cpu) > + cpu = virCPUDefCopy(cpu); > + else > + cpu = virCPUDefCopy(vm->def->cpu); > + > + if (!cpu) > return -1; First two reads were very confusing as this is overwriting the argument .... > > - VIR_DEBUG("Replacing CPU def with the updated one"); > + VIR_DEBUG("Replacing CPU definition"); > > *origCPU = vm->def->cpu; > vm->def->cpu = cpu; ... just to store it here. At this point it's no longer necessary. You can first move the old def into 'origCPU' and then directly assign into the variable to avoid the overwrite for readability.

> @@ -11064,13 +11075,6 @@ qemuDomainFixupCPUs(virDomainObj *vm, > !vm->def->cpu->model) > return 0; > > - /* Missing origCPU means QEMU created exactly the same virtual CPU which > - * we asked for or libvirt was too old to mess up the translation from > - * host-model. > - */ > - if (!*origCPU) > - return 0; This function is called from two places: src/qemu/qemu_process.c=qemuProcessStartWithMemoryState(virConnectPtr conn, src/qemu/qemu_process.c: qemuDomainFixupCPUs(vm, &cookie->cpu) < 0)

src/qemu/qemu_process.c=qemuProcessRefreshCPU(virQEMUDriver *driver, src/qemu/qemu_process.c: if (qemuDomainFixupCPUs(vm, &priv->origCPU) < 0 'priv->origCPU' which is used in the latter is AFAIU filled only in 'qemuDomainUpdateCPU' called when starting up a qemu process and in 'qemuDomainObjPrivateXMLParse' when loading a status XML. In both cases there are code paths which may leave 'priv->origCPU' to be NULL. The following code will dereference it unconditionally: if (virCPUDefFindFeature(*origCPU, "cmt")) {

> diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c > index e4bcb628cf..8165c28dbc 100644 > --- a/src/qemu/qemu_process.c > +++ b/src/qemu/qemu_process.c > @@ -4449,19 +4447,11 @@ qemuProcessUpdateLiveGuestCPU(virDomainObj *vm, > !def->cpu->model)) > return 0; > > - orig = virCPUDefCopy(def->cpu); > - > - if ((rc = virCPUUpdateLive(def->os.arch, def->cpu, enabled, disabled)) < 0) { > + if ((rc = virCPUUpdateLive(def->os.arch, def->cpu, enabled, disabled)) < 0) > return -1; > - } else if (rc == 0) { > - /* Store the original CPU in priv if QEMU changed it and we didn't > - * get the original CPU via migration, restore, or snapshot revert. > - */ > - if (!priv->origCPU && !virCPUDefIsEqual(def->cpu, orig, false)) > - priv->origCPU = g_steal_pointer(&orig); > > + if (rc == 0) > def->cpu->check = VIR_CPU_CHECK_FULL; > - } This code path can also theoretically be called on a status XML created by libvirt which didn't unconditionally fill origCPU IIUC.