-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 <imammedo(a)redhat.com&gt; Reviewed-by: Andrew Jones <drjones(a)redhat.com&gt; Reviewed-by: Eduardo Habkost <ehabkost(a)redhat.com&gt; --- v7: - add corrections to deprication doc formulas/math/section title - drop Note section (Eduardo Habkost <ehabkost(a)redhat.com>) v6: - s/socket/sockets/, the same for core, thread in subsection (Andrew Jones <drjones(a)redhat.com>) v5: - extend deprecation doc, adding that maxcpus should be multiple of present on CLI [sockets/cores/threads] options (Eduardo Habkost <ehabkost(a)redhat.com>) v4: - missed dot comment, fix it with s/./,/ (Andrew Jones <drjones(a)redhat.com>) v3: - more spelling fixes (Andrew Jones <drjones(a)redhat.com>) - place deprecation check after (sockets * cores * threads > max_cpus) check (Eduardo Habkost <ehabkost(a)redhat.com>) v2: - spelling&&co fixes (Andrew Jones <drjones(a)redhat.com>) --- qemu-deprecated.texi | 13 +++++++++++++ vl.c | 7 +++++++ 2 files changed, 20 insertions(+) diff --git a/qemu-deprecated.texi b/qemu-deprecated.texi index 1b9c007..31dc1b3 100644 --- a/qemu-deprecated.texi +++ b/qemu-deprecated.texi @@ -139,6 +139,19 @@ 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. +@subsection -smp (invalid topologies) (since 3.1) + +CPU topology properties should describe whole machine topology including +possible CPUs. In other words: @var{maxcpus} should be equal to +@math{@var{sockets} * @var{cores} * @var{threads}}. + +However, historically it was possible to start QEMU with an incorrect topology +where @math{@var{n} <= @var{sockets} * @var{cores} * @var{threads} < @var{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. + @math{@var{sockets} * @var{cores} * @var{threads} = @var{maxcpus}}. + @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

commit (5cdc9b76e3 vl.c: Remove dead assignment) removed sockets calculation when 'sockets' weren't provided on CLI since there wasn't any users for it back then. Exiting checks are neither reachable } else if (sockets * cores * threads < cpus) { or nor triggable if (sockets * cores * threads > max_cpus) so we weren't noticing wrong topology since then, since users recalculate sockets adhoc on their own. However with deprecation check it becomes noticable, for example -smp 2 will start printing warning: "warning: Invalid CPU topology deprecated: sockets (1) * cores (1) * threads (1) != maxcpus (2)" calculating sockets if they weren't specified. Fix it by returning back sockets calculation if it's omited on CLI. Signed-off-by: Igor Mammedov <imammedo(a)redhat.com&gt; Reviewed-by: Andrew Jones <drjones(a)redhat.com&gt; --- vl.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/vl.c b/vl.c index 7fd700e..9e56696 100644 --- a/vl.c +++ b/vl.c @@ -1210,11 +1210,14 @@ static void smp_parse(QemuOpts *opts) /* compute missing values, prefer sockets over cores over threads */ if (cpus == 0 || sockets == 0) { - sockets = sockets > 0 ? sockets : 1; cores = cores > 0 ? cores : 1; threads = threads > 0 ? threads : 1; if (cpus == 0) { + sockets = sockets > 0 ? sockets : 1; cpus = cores * threads * sockets; + } else { + max_cpus = qemu_opt_get_number(opts, "maxcpus", cpus); + sockets = max_cpus / (cores * threads); } } else if (cores == 0) { threads = threads > 0 ? threads : 1; -- 2.7.4