machine: Deprecate "parameter=0" SMP configurations

In the SMP configuration, we should either provide a topology
parameter with a reasonable value (greater than zero) or just
omit it and QEMU will compute the missing value.

The users shouldn't provide a configuration with any parameter
of it specified as zero (e.g. -smp 8,sockets=0) which could
possibly cause unexpected results in the -smp parsing. So we
deprecate this kind of configurations since 6.2 by adding the
explicit sanity check.

Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20210929025816.21076-3-wangyanan55@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

3 files changed, 36 insertions, 5 deletions

diff --git a/docs/about/deprecated.rst b/docs/about/deprecated.rst
index 2f7db9a98d..0bed6ecb1d 100644
--- a/docs/about/deprecated.rst
+++ b/docs/about/deprecated.rst
@@ -160,6 +160,21 @@ Use ``-display sdl`` instead.
 
 Use ``-display curses`` instead.
 
+``-smp`` ("parameter=0" SMP configurations) (since 6.2)
+'''''''''''''''''''''''''''''''''''''''''''''''''''''''
+
+Specified CPU topology parameters must be greater than zero.
+
+In the SMP configuration, users should either provide a CPU topology
+parameter with a reasonable value (greater than zero) or just omit it
+and QEMU will compute the missing value.
+
+However, historically it was implicitly allowed for users to provide
+a parameter with zero value, which is meaningless and could also possibly
+cause unexpected results in the -smp parsing. So support for this kind of
+configurations (e.g. -smp 8,sockets=0) is deprecated since 6.2 and will
+be removed in the near future, users have to ensure that all the topology
+members described with -smp are greater than zero.
 
 Plugin argument passing through ``arg=<string>`` (since 6.1)
 ''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
diff --git a/hw/core/machine.c b/hw/core/machine.c
index 067f42b528..4e409261c9 100644
--- a/hw/core/machine.c
+++ b/hw/core/machine.c
@@ -835,6 +835,20 @@ static void machine_set_smp(Object *obj, Visitor *v, const char *name,
         return;
     }
 
+    /*
+     * Specified CPU topology parameters must be greater than zero,
+     * explicit configuration like "cpus=0" is not allowed.
+     */
+    if ((config->has_cpus && config->cpus == 0) ||
+        (config->has_sockets && config->sockets == 0) ||
+        (config->has_dies && config->dies == 0) ||
+        (config->has_cores && config->cores == 0) ||
+        (config->has_threads && config->threads == 0) ||
+        (config->has_maxcpus && config->maxcpus == 0)) {
+        warn_report("Deprecated CPU topology (considered invalid): "
+                    "CPU topology parameters must be greater than zero");
+    }
+
     mc->smp_parse(ms, config, errp);
     if (*errp) {
         goto out_free;
diff --git a/qemu-options.hx b/qemu-options.hx
index ceca52818a..4a092a092a 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -233,11 +233,13 @@ SRST
     of computing the CPU maximum count.
 
     Either the initial CPU count, or at least one of the topology parameters
-    must be specified. Values for any omitted parameters will be computed
-    from those which are given. Historically preference was given to the
-    coarsest topology parameters when computing missing values (ie sockets
-    preferred over cores, which were preferred over threads), however, this
-    behaviour is considered liable to change.
+    must be specified. The specified parameters must be greater than zero,
+    explicit configuration like "cpus=0" is not allowed. Values for any
+    omitted parameters will be computed from those which are given.
+    Historically preference was given to the coarsest topology parameters
+    when computing missing values (ie sockets preferred over cores, which
+    were preferred over threads), however, this behaviour is considered
+    liable to change.
 ERST
 
 DEF("numa", HAS_ARG, QEMU_OPTION_numa,