Re: [PATCH v3] KVM: x86: Add CONFIG_KVM_MAX_NR_VCPUS

["Meyer, Kyle" <kyle.meyer@hpe.com>]

> > Add a Kconfig entry to set the maximum number of vCPUs per KVM guest and
> > set the default value to 4096 when MAXSMP is enabled.
>
> I'd like to capture why the max is set to 4096, both the justification and why
> we don't want to go further at this point.
>
> If you've no objection, I'll massage the changelog to this when applying:
>
>   Add a Kconfig entry to set the maximum number of vCPUs per KVM guest and
>   set the default value to 4096 when MAXSMP is enabled, as there are use
>   cases that want to create more than the currently allow 1024 vCPUs and
>   are more than happy to eat the memory overhead.
>
>   The Hyper-V TLFS doesn't allow more than 64 sparse banks, i.e. allows a
>   maximum of 4096 virtual CPUs. Cap KVM's maximum number of virtual CPUs
>   to 4096 to avoid exceeding Hyper-V's limit as KVM support for Hyper-V is
>   unconditional, and alternatives like dynamically disabling Hyper-V
>   enlightenments that rely on sparse banks would require non-trivial code
>   changes.
>
> > Suggested-by: Sean Christopherson <seanjc@google.com>
> > Signed-off-by: Kyle Meyer <kyle.meyer@hpe.com>
> > ---
> > v2 -> v3: Default KVM_MAX_VCPUS to 1024 when CONFIG_KVM_MAX_NR_VCPUS is not
> > defined. This prevents build failures in arch/x86/events/intel/core.c and
> > drivers/vfio/vfio_main.c when KVM is disabled.
> > 
> >  arch/x86/include/asm/kvm_host.h |  4 ++++
> >  arch/x86/kvm/Kconfig            | 11 +++++++++++
> >  2 files changed, 15 insertions(+)
> > 
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 3bc146dfd38d..cd27e0a00765 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -39,7 +39,11 @@
> >  
> >  #define __KVM_HAVE_ARCH_VCPU_DEBUGFS
> >
>
> And another thing I'll add if you don't object is a comment to explain that this
> is purely to play nice with CONFIG_KVM=n.  And FWIW, I hope to make this go away
> entirely: https://lore.kernel.org/all/20230916003118.2540661-27-seanjc@google.com
>
> /*
>  * CONFIG_KVM_MAX_NR_VCPUS is defined iff CONFIG_KVM!=n, provide a dummy max if
>  * KVM is disabled (arbitrarily use default from CONFIG_KVM_MAX_NR_VCPUS).
>  */ 
>
> > +#ifdef CONFIG_KVM_MAX_NR_VCPUS
> > +#define KVM_MAX_VCPUS CONFIG_KVM_MAX_NR_VCPUS
> > +#else
> >  #define KVM_MAX_VCPUS 1024
> > +#endif
> >  
> >  /*
> >   * In x86, the VCPU ID corresponds to the APIC ID, and APIC IDs
> > diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
> > index 89ca7f4c1464..e730e8255e22 100644
> > --- a/arch/x86/kvm/Kconfig
> > +++ b/arch/x86/kvm/Kconfig
> > @@ -141,4 +141,15 @@ config KVM_XEN
> >  config KVM_EXTERNAL_WRITE_TRACKING
> >  	bool
> >  
> > +config KVM_MAX_NR_VCPUS
> > +	int "Maximum number of vCPUs per KVM guest"
> > +	depends on KVM
> > +	range 1024 4096
> > +	default 4096 if MAXSMP
> > +	default 1024
> > +	help
> > +	  Set the maximum number of vCPUs per KVM guest. Larger values will increase
> > +	  the memory footprint of each KVM guest, regardless of how many vCPUs are
> > +	  configured.
>
> Last nit, I think the last linke should be like so:
>
>        the memory footprint of each KVM guest, regardless of how many vCPUs are
>        created for a given VM.
>
> No need for a v4 unless you object to any of the above, I'm happt to fixup when
> applying.

LGTM. I have no objections to any of those changes. Thank you very much.