[PULL 1/1] machine: Disallow specifying topology parameters as zero

[Eduardo Habkost <ehabkost@redhat.com>]

From: Yanan Wang <wangyanan55@huawei.com>

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. Users should
have never provided a configuration with parameters as zero
(e.g. -smp 8,sockets=0) which should be treated as invalid.

But commit 1e63fe68580 (machine: pass QAPI struct to mc->smp_parse)
has added some doc which implied that 'anything=0' is valid and
has the same semantics as omitting a parameter.

To avoid meaningless configurations possibly introduced by users
in the future and consequently a necessary deprecation process,
fix the doc and also add the corresponding sanity check.

Fixes: 1e63fe68580 (machine: pass QAPI struct to mc->smp_parse)
Suggested-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Tested-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <20210816024522.143124-2-wangyanan55@huawei.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
---
 hw/core/machine.c | 14 ++++++++++++++
 qapi/machine.json |  6 +++---
 qemu-options.hx   | 12 +++++++-----
 3 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/hw/core/machine.c b/hw/core/machine.c
index 54e040587dd..a7e119469aa 100644
--- a/hw/core/machine.c
+++ b/hw/core/machine.c
@@ -832,6 +832,20 @@ static void machine_set_smp(Object *obj, Visitor *v, const char *name,
         return;
     }
 
+    /*
+     * A specified topology parameter 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)) {
+        error_setg(errp, "CPU topology parameters must be greater than zero");
+        goto out_free;
+    }
+
     mc->smp_parse(ms, config, errp);
     if (*errp) {
         goto out_free;
diff --git a/qapi/machine.json b/qapi/machine.json
index c3210ee1fb2..9272cb3cf8b 100644
--- a/qapi/machine.json
+++ b/qapi/machine.json
@@ -1288,8 +1288,8 @@
 ##
 # @SMPConfiguration:
 #
-# Schema for CPU topology configuration.  "0" or a missing value lets
-# QEMU figure out a suitable value based on the ones that are provided.
+# Schema for CPU topology configuration. A missing value lets QEMU
+# figure out a suitable value based on the ones that are provided.
 #
 # @cpus: number of virtual CPUs in the virtual machine
 #
@@ -1297,7 +1297,7 @@
 #
 # @dies: number of dies per socket in the CPU topology
 #
-# @cores: number of cores per thread in the CPU topology
+# @cores: number of cores per die in the CPU topology
 #
 # @threads: number of threads per core in the CPU topology
 #
diff --git a/qemu-options.hx b/qemu-options.hx
index 83aa59a920f..aee622f577d 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -227,11 +227,13 @@ 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,
-- 
2.31.1