** Description changed:
+ [ Impact ]
+
+ QEMU users on Ubuntu Jammy who try to spawn a VM with more than 288
+ vCPUs will not be able to do so, because the machine types available
+ don't support such scenario. The following error will happen:
+
+ qemu-system-x86_64: Invalid SMP CPUs 300. The max CPUs supported by
+ machine 'pc-q35-jammy' is 288
+
+ [ Test Plan ]
+
+ Ideally, the test should be performed in a machine with more than 288
+ physical CPUs available. However, due to the difficulty in finding such
+ systems, it is possible to emulate the usage of more than 288 vCPUs.
+
+ On a Jammy machine, you can do:
+
+ $ sudo qemu-system-x86_64 -M pc-q35-jammy,accel=kvm,kernel-irqchip=split
+ -device intel-iommu,intremap=on -smp cpus=300,maxcpus=300 -enable-kvm
+ -net none -m 4096M -nographic -kernel /boot/vmlinuz -initrd
+ /boot/initrd.img -chardev stdio,mux=on,id=char0 -mon
+ chardev=char0,mode=readline -serial chardev:char0 -append
+ "console=ttyS0"
+
+ You will notice that the command will fail, as expected.
+
+ The proposed fix is to create a new machine type on Jammy, in order to
+ minimize the possibility of regressions in deployments using the
+ existing machine types. This new type is named pc-{q35,i440fx}-jammy-
+ maxcpus. When doing the test, make sure to provide this new machine
+ type (as part of the "-M" argument).
+
+ [ Where problems could occur ]
+
+ As explained above, a new machine type was created in order to minimize
+ the possibility of regressions. As such, the existing
+ "pc-{q35,i440fx}-jammy" machine types should continue to work as before,
+ without any change.
+
+ [ Original Description ]
+
During testing of an AMD Genoa CPU, it was discovered that qemu-system-
amd64 doesn't support enough cpus.
The specific error the tester received was:
qemu-system-x86_64: Invalid SMP CPUs 384. The max supported by machine
'pc-q35-7.1' is 288
Looking at the sournce that seems to be an easy fix at first glance:
https://github.com/qemu/qemu/blob/master/hw/i386/pc_q35.c
372 machine_class_allow_dynamic_sysbus_dev(m, TYPE_VMBUS_BRIDGE);
373 m->max_cpus = 288;
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https://bugs.launchpad.net/bugs/2012763
Title:
qemu-system-amd64 max cpus is too low for latest processors
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