On 9/28/22 20:11, Daniel P. Berrangé wrote:
On Fri, Sep 02, 2022 at 09:55:22AM +0200, Pierre Morel wrote:
S390x do not support multithreading in the guest.
Do not let admin falsely specify multithreading on QEMU
smp commandline.
Signed-off-by: Pierre Morel <pmorel@xxxxxxxxxxxxx>
---
hw/s390x/s390-virtio-ccw.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/hw/s390x/s390-virtio-ccw.c b/hw/s390x/s390-virtio-ccw.c
index 70229b102b..b5ca154e2f 100644
--- a/hw/s390x/s390-virtio-ccw.c
+++ b/hw/s390x/s390-virtio-ccw.c
@@ -86,6 +86,9 @@ static void s390_init_cpus(MachineState *machine)
MachineClass *mc = MACHINE_GET_CLASS(machine);
int i;
+ /* Explicitely do not support threads */
+ assert(machine->smp.threads == 1);
What is the functional effect for currently released QEMU versions
if a user has set threads == 2 for an s390 machine ? Is the
threads setting simply ignored ?
It is not ignored, the number of CPUs per sockets seen by the guest is
cores*threads
If we want to eliminate this mistake, then there's two possible
options
* If it had no effect, treat this like a deprecation process
where we print a warning for 2 releases, and then turn the
warning into an error. Gives a little grace to fix the config
mistakes some users might have made, at a time convenient to
them.
Or
* If it had effect and we need migration compatibility then forbid
threads > 1 only for new machine type versions, so existing
deployed guests are not changed.
With regards,
Daniel
Thanks for your comments Daniel.
I will need to forbid threads > 1 for new machine.
regards,
Pierre
--
Pierre Morel
IBM Lab Boeblingen
