pc: Don't make die-id mandatory unless necessary

We have this issue reported when using libvirt to hotplug CPUs:
https://bugzilla.redhat.com/show_bug.cgi?id=1741451

Basically, libvirt is not copying die-id from
query-hotpluggable-cpus, but die-id is now mandatory.

We could blame libvirt and say it is not following the documented
interface, because we have this buried in the QAPI schema
documentation:

> Note: currently there are 5 properties that could be present
> but management should be prepared to pass through other
> properties with device_add command to allow for future
> interface extension. This also requires the filed names to be kept in
> sync with the properties passed to -device/device_add.

But I don't think this would be reasonable from us.  We can just
make QEMU more flexible and let die-id to be omitted when there's
no ambiguity.  This will allow us to keep compatibility with
existing libvirt versions.

Test case included to ensure we don't break this again.

Fixes: commit 176d2cda0dee ("i386/cpu: Consolidate die-id validity in smp context")
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20190816170750.23910-1-ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index 3494423d63..c7200b0b54 100644
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -2421,6 +2421,14 @@ static void pc_cpu_pre_plug(HotplugHandler *hotplug_dev,
         int max_socket = (ms->smp.max_cpus - 1) /
                                 smp_threads / smp_cores / pcms->smp_dies;
 
+        /*
+         * die-id was optional in QEMU 4.0 and older, so keep it optional
+         * if there's only one die per socket.
+         */
+        if (cpu->die_id < 0 && pcms->smp_dies == 1) {
+            cpu->die_id = 0;
+        }
+
         if (cpu->socket_id < 0) {
             error_setg(errp, "CPU socket-id is not set");
             return;