Performing the verification on Mantic.

First, verifying that we can reproduce the problem.

# apt policy qemu-system-x86
qemu-system-x86:
  Installed: 1:8.0.4+dfsg-1ubuntu3.23.10.3
  Candidate: 1:8.0.4+dfsg-1ubuntu3.23.10.3
  Version table:
 *** 1:8.0.4+dfsg-1ubuntu3.23.10.3 500
        500 http://archive.ubuntu.com/ubuntu mantic-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     1:8.0.4+dfsg-1ubuntu3.23.10.2 500
        500 http://security.ubuntu.com/ubuntu mantic-security/main amd64 Packages
     1:8.0.4+dfsg-1ubuntu3 500
        500 http://archive.ubuntu.com/ubuntu mantic/main amd64 Packages

# 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"
qemu-system-x86_64: Invalid SMP CPUs 300. The max CPUs supported by machine 'pc-q35-jammy' is 288

# qemu-system-x86_64 -M pc-q35-mantic,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"
qemu-system-x86_64: Invalid SMP CPUs 300. The max CPUs supported by machine 'pc-q35-mantic' is 288

Now, updating the package and verifying that the version from -proposed fixes the issue:

# apt policy qemu-system-x86
qemu-system-x86:
  Installed: 1:8.0.4+dfsg-1ubuntu3.23.10.5
  Candidate: 1:8.0.4+dfsg-1ubuntu3.23.10.5
  Version table:
 *** 1:8.0.4+dfsg-1ubuntu3.23.10.5 100
        100 http://archive.ubuntu.com/ubuntu mantic-proposed/main amd64 Packages
        100 /var/lib/dpkg/status
     1:8.0.4+dfsg-1ubuntu3.23.10.3 500
        500 http://archive.ubuntu.com/ubuntu mantic-updates/main amd64 Packages
     1:8.0.4+dfsg-1ubuntu3.23.10.2 500
        500 http://security.ubuntu.com/ubuntu mantic-security/main amd64 Packages
     1:8.0.4+dfsg-1ubuntu3 500
        500 http://archive.ubuntu.com/ubuntu mantic/main amd64 Packages

# qemu-system-x86_64 -M pc-q35-jammy-maxcpus,accel=kvm,kernel-irqchip=split -device intel-iommu,intremap=on -smp cpus=300,maxcpus=300 -enable-kvm -net none -m 4096M -nographic -kern
el /boot/vmlinuz -initrd /boot/initrd.img -chardev stdio,mux=on,id=char0 -mon chardev=char0,mode=readline -serial chardev:char0 -append "console=ttyS0"
qemu-system-x86_64: warning: Number of SMP cpus requested (300) exceeds the recommended cpus supported by KVM (12)
qemu-system-x86_64: warning: Number of hotpluggable cpus requested (300) exceeds the recommended cpus supported by KVM (12)
...
<no QEMU error>

# qemu-system-x86_64 -M pc-q35-mantic-maxcpus,accel=kvm,kernel-irqchip=split -device intel-iommu,intremap=on -smp cpus=300,maxcpus=300 -enable-kvm -net none -m 4096M -nographic -kern
el /boot/vmlinuz -initrd /boot/initrd.img -chardev stdio,mux=on,id=char0 -mon chardev=char0,mode=readline -serial chardev:char0 -append "console=ttyS0"
qemu-system-x86_64: warning: Number of SMP cpus requested (300) exceeds the recommended cpus supported by KVM (12)
qemu-system-x86_64: warning: Number of hotpluggable cpus requested (300) exceeds the recommended cpus supported by KVM (12)
...
<no QEMU error>


This concludes the verification on Mantic.