Re: [PATCH] KVM: x86: Increase KVM_MAX_VCPUS to 2048

[Vitaly Kuznetsov <vkuznets@redhat.com>]

David Matlack <dmatlack@google.com> writes:

> On Mon, Jun 13, 2022 at 11:35 AM Kyle Meyer <kyle.meyer@hpe.com> wrote:
>>
>> Increase KVM_MAX_VCPUS to 2048 so we can run larger virtual machines.
>
> Does the host machine have 2048 CPUs (or more) as well in your usecase?
>
> I'm wondering if it makes sense to start configuring KVM_MAX_VCPUS
> based on NR_CPUS. That way KVM can scale up on large machines without
> using more memory on small machines.
>
> e.g.
>
> /* Provide backwards compatibility. */
> #if NR_CPUS < 1024
>   #define KVM_MAX_VCPUS 1024
> #else
>   #define KVM_MAX_VCPUS NR_CPUS
> #endif
>
> The only downside I can see for this approach is if you are trying to
> kick the tires a new large VM on a smaller host because the new "large
> host" hardware hasn't landed yet.

FWIW, while I don't think there's anything wrong with such approach, it
won't help much distro kernels which are not recompiled to meet the
needs of a particular host. According to Kyle's numbers, the biggest
growth is observed with 'struct kvm_ioapic' and that's only because of
'struct rtc_status' embedded in it. Maybe it's possible to use something
different from a KVM_MAX_VCPU_IDS-bound flat bitmask there? I'm not sure
how important this is as it's just another 4K per-VM and when guest's
memory is taken into account it's probably not much.

The growth in 'struct kvm'/'struct kvm_arch' seems to be insignificant
and on-stack allocations are probably OK.

-- 
Vitaly