[Qemu-devel] [PATCH v5 1/2] vl.c deprecate incorrect CPUs topology

[Igor Mammedov]

-smp [cpus],sockets/cores/threads[,maxcpus] should describe topology
so that total number of logical CPUs [sockets * cores * threads]
would be equal to [maxcpus], however historically we didn't have
such check in QEMU and it is possible to start VM with an invalid
topology.
Deprecate invalid options combination so we can make sure that
the topology VM started with is always correct in the future.
Users with an invalid sockets/cores/threads/maxcpus values should
fix their CLI to make sure that
   [sockets * cores * threads] == [maxcpus]

Signed-off-by: Igor Mammedov <address@hidden>
---
v5:
  - extend deprecation doc, adding that maxcpus should be multiple of
    present on CLI [sockets/cores/threads] options
    (Eduardo Habkost <address@hidden>)
v4:
  - missed dot comment, fix it with s/./,/ (Andrew Jones <address@hidden>)
v3:
  - more spelling fixes (Andrew Jones <address@hidden>)
  - place deprecation check after (sockets * cores * threads > max_cpus) check
    (Eduardo Habkost <address@hidden>)
v2:
  - spelling&&co fixes (Andrew Jones <address@hidden>)
---
 qemu-deprecated.texi | 19 +++++++++++++++++++
 vl.c                 |  7 +++++++
 2 files changed, 26 insertions(+)

diff --git a/qemu-deprecated.texi b/qemu-deprecated.texi
index 87212b6..827c3ce 100644
--- a/qemu-deprecated.texi
+++ b/qemu-deprecated.texi
@@ -159,6 +159,25 @@ The 'file' driver for drives is no longer appropriate for 
character or host
 devices and will only accept regular files (S_IFREG). The correct driver
 for these file types is 'host_cdrom' or 'host_device' as appropriate.
 
address@hidden -smp X,[socket=a,core=b,thread=c],maxcpus=Y (since 3.1)
+
+CPU topology properties should describe whole machine topology including
+possible CPUs, but historically it was possible to start QEMU with
+an incorrect topology where
+  sockets * cores * threads >= X && X < maxcpus
+which could lead to an incorrect topology enumeration by the guest.
+Support for invalid topologies will be removed, the user must ensure
+topologies described with -smp include all possible cpus, i.e.
+  sockets * cores * threads == maxcpus
+Note: it's assumed that maxcpus value must be multiple of the topology
+options present on command line to avoid creating an invalid topology.
+If maxcpus isn't be multiple of present topology options then the condition
+(sockets * cores * threads == maxcpus) can't be satisfied and it will
+trigger deprecation warning which later will be converted to a error.
+If you get deprecation warning it's recommended to explicitly specify
+a correct topology to make warning go away and ensure that it will
+continue working in the future.
+
 @section QEMU Machine Protocol (QMP) commands
 
 @subsection block-dirty-bitmap-add "autoload" parameter (since 2.12.0)
diff --git a/vl.c b/vl.c
index 5ba06ad..7fd700e 100644
--- a/vl.c
+++ b/vl.c
@@ -1246,6 +1246,13 @@ static void smp_parse(QemuOpts *opts)
             exit(1);
         }
 
+        if (sockets * cores * threads != max_cpus) {
+            warn_report("Invalid CPU topology deprecated: "
+                        "sockets (%u) * cores (%u) * threads (%u) "
+                        "!= maxcpus (%u)",
+                        sockets, cores, threads, max_cpus);
+        }
+
         smp_cpus = cpus;
         smp_cores = cores;
         smp_threads = threads;
-- 
2.7.4