How about increasing max_cpus for q35 ?

[Dario Faggioli]

Hello,

Sorry for the potentially naive question, but I'm not clear what the
process would be if, say, I'd like to raise the number of maximum CPUs
a q35 VM can have.

So, right now we have:

void pc_q35_2_7_machine_options(MachineClass *m) {
  ...
  m->max_cpus = 255;
}

And:

void pc_q35_machine_options(MachineClass *m)
{
  ...
  m->max_cpus = 288;
}

Focusing on the latter, it comes from this commit:

https://gitlab.com/qemu-project/qemu/-/commit/00d0f9fd6602a27b204f672ef5bc8e69736c7ff1
  
  pc: q35: Bump max_cpus to 288

  Along with it for machine versions 2.7 and older keep
  it at 255.

So, it was 255 and is now 288. This seems to me to be there since QEMU
2.8.0.

Now, as far as I understand, KVM can handle 1024, at least since this
commit (and a couple of other related ones):

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=074c82c8f7cf8a46c3b81965f122599e3a133450
"kvm: x86: Increase MAX_VCPUS to 1024"

Which basically does:

-#define KVM_MAX_VCPUS 288
+#define KVM_MAX_VCPUS 1024

And it's included in kernels >= 5.15.

So, what's the correct way of bumping up the limit again? Just changing
that assignment in pc_q35_machine_options() ? Or do we want a new
version of the machine type or something like that?

Thanks and Regards
-- 
Dario Faggioli, Ph.D
http://about.me/dario.faggioli
Virtualization Software Engineer
SUSE Labs, SUSE https://www.suse.com/
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