Re: [PATCH 5/5] qemuProcessUpdateCPU: do not change 'fallback' to ALLOW for pSeries guests

Commit v3.10.0-182-g237f045d9a ("qemu: Ignore fallback CPU attribute on reconnect") forced CPU 'fallback' to ALLOW, regardless of user choice. This fixed a situation in which guests created with older Libvirt versions, which used CPU mode 'host-model' in runtime, would fail to launch in a newer Libvirt if the fallback was set to FORBID. This would lead to a scenario where the CPU was translated to 'host-model' to 'custom', but then the FORBID setting would make the translation process fail. This fix has a side effect for PSeries guests. PSeries can operate with 'host-model' in runtime due to specific PPC64 mechanics regarding compatibility mode. In fact, the update() implementation of the cpuDriverPPC64 driver is a NO-OP if CPU mode is 'host-model', and the driver does not implement translate(). The result is that PSeries guests aren't affected by the problem, but they are being affected by the fix - users are seeing 'fallback' mode being changed without necessity during daemon restart. All other cpuArchDrivers implements update() and changes guest mode to VIR_CPU_MODE_CUSTOM, meaning that PSeries is currently the only exception to this logic. Let's make it official. https://bugzilla.redhat.com/show_bug.cgi?id=1660711 CC: Jiri Denemark <jdenemar(a)redhat.com&gt; Signed-off-by: Daniel Henrique Barboza <danielhb413(a)gmail.com&gt; --- src/qemu/qemu_process.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c index a1ef1d42b0..fec1720f33 100644 --- a/src/qemu/qemu_process.c +++ b/src/qemu/qemu_process.c @@ -4347,8 +4347,14 @@ qemuProcessUpdateCPU(virQEMUDriverPtr driver, /* The host CPU model comes from host caps rather than QEMU caps so * fallback must be allowed no matter what the user specified in the XML. + * + * Note: PSeries domains are able to run with host-model CPU by design, + * even on Libvirt newer than 2.3, never replacing host-model with + * custom in the virCPUUpdate() call prior to this function. It is not + * needed to change the user defined 'fallback' attribute in this case. */ - vm->def->cpu->fallback = VIR_CPU_FALLBACK_ALLOW; + if (!qemuDomainIsPSeries(vm->def)) + vm->def->cpu->fallback = VIR_CPU_FALLBACK_ALLOW; if (qemuProcessFetchGuestCPU(driver, vm, asyncJob, &cpu, &disabled) < 0) return -1;