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

[Sean Christopherson <seanjc@google.com>]

On Thu, Jun 16, 2022, David Matlack wrote:
> On Tue, Jun 14, 2022 at 1:28 AM Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
> >
> > 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.
> 
> Heh. My point here doesn't make sense. The actual number of CPUs in
> the host machine wouldn't matter, just the host kernel's NR_CPUS.
> 
> >
> > 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.
> 
> But is there a use-case for running a VM with more vCPUs than the
> kernel's NR_CPUS?

With custom configs yes, e.g. my host kernel config for my anemic old system has
CONFIG_NR_CPUS=12 and I occasionally run VMs with >12 vCPUs for testing purposes.

But that obviously doesn't apply if KVM_MAX_VCPUS has a lower bound of 1024, so
I agree that making KVM_MAX_VCPUS=max(1024, NR_CPUS) should do just fine.