On Fri, 29 Jan 2016 13:36:05 -0200 Eduardo Habkost <ehabk...@redhat.com> wrote:
> On Fri, Jan 29, 2016 at 04:10:47PM +0100, Igor Mammedov wrote: > > On Fri, 29 Jan 2016 12:24:18 -0200 > > Eduardo Habkost <ehabk...@redhat.com> wrote: > > > > > On Fri, Jan 29, 2016 at 02:52:30PM +1100, David Gibson wrote: > > > > On Thu, Jan 28, 2016 at 11:19:43AM +0530, Bharata B Rao wrote: > > > > > Prevent guests from booting with CPU topologies that have partially > > > > > filled CPU cores or can result in partially filled CPU cores after > > > > > CPU hotplug like > > > > > > > > > > -smp 15,sockets=1,cores=4,threads=4,maxcpus=16 or > > > > > -smp 15,sockets=1,cores=4,threads=4,maxcpus=17. > > > > > > > > > > This is enforced by introducing MachineClass::validate_smp_config() > > > > > that gets called from generic SMP parsing code. Machine type versions > > > > > that want to enforce this can define this to the generic version > > > > > provided. > > > > > > > > > > Only sPAPR and PC machine types starting from version 2.6 enforce > > > > > this in > > > > > this patch. > > > > > > > > > > Signed-off-by: Bharata B Rao <bhar...@linux.vnet.ibm.com> > > > > > > > > I've been kind of lost in the back and forth about > > > > threads/cores/sockets. > > > > > > > > What, in the end, is the rationale for allowing partially filled > > > > sockets, but not partially filled cores? > > > > > > I don't think there's a good reason for that (at least for PC). > > > > > > It's easier to relax the requirements later if necessary, than > > > dealing with compatibility issues again when making the code more > > > strict. So I suggest we make validate_smp_config_generic() also > > > check if smp_cpus % (smp_threads * smp_cores) == 0. > > > > that would break exiting setups. > > Not if we do that only on newer machine classes. > validate_smp_config_generic() will be used only on *-2.6 and > newer. > > > > > > Also in case of cpu hotplug this patch will break migration > > as target QEMU might refuse starting with hotplugged CPU thread. > > This won't change older machine-types. > > But I think you are right: it can break migration on pc-2.6, too. > But: isn't migration already broken when creating other sets of > CPUs that can't represented using -smp? > > How exactly would you migrate a machine today, if you run: > > $ qemu-system-x86_64 -smp 16,sockets=2,cores=2,threads=2,maxcpus=32 > (QMP) cpu-add id=31 that's invalid topology and should exit with error at start-up, however it shouldn't be smp_cpus vs sockets,cores,threads check but rather max_cpus vs sockets,cores,threads,maxcpus check. something like this: diff --git a/vl.c b/vl.c index f043009..3afa0b6 100644 --- a/vl.c +++ b/vl.c @@ -1239,9 +1239,9 @@ static void smp_parse(QemuOpts *opts) } max_cpus = qemu_opt_get_number(opts, "maxcpus", cpus); - if (sockets * cores * threads > max_cpus) { - error_report("cpu topology: " - "sockets (%u) * cores (%u) * threads (%u) > " + if (sockets * cores * threads == max_cpus) { + error_report("invalid cpu topology: " + "sockets (%u) * cores (%u) * threads (%u) not equal " "maxcpus (%u)", sockets, cores, threads, max_cpus); exit(1); > > > > > > Perhaps this check should be enforced per target/machine if > > arch requires it. > > It is. Please see the patch. It introduces a validate_smp_config > method. > > But we need your input to clarify if > validate_smp_config_generic() is safe for pc-2.6 too. it breaks migration as it could prevent target from starting if there is hotplugged CPUs on source side.