Re: How about increasing max_cpus for q35 ?

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 9 Nov 2022 13:36:07 +0000
Dario Faggioli <dfaggioli@suse.com> wrote:

> 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()

that and preserve 288 limit for existing machine types.

Basically the same as above QEMU commit with difference
that pc_q35_2_8_machine_options() should be replaced by
7.2 equivalent.


PS:
we are still missing OVMF support for 1024,
but it's being worked on.

> ? Or do we want a new
> version of the machine type or something like that?
> 
> Thanks and Regards