Re: [PATCH 26/29] KVM: x86: enable up to 16 planes

[Sean Christopherson <seanjc@google.com>]

On Tue, Apr 01, 2025, Paolo Bonzini wrote:
> Allow up to 16 VM planes, it's a nice round number.
> 
> FIXME: online_vcpus is used by x86 code that deals with TSC synchronization.
> Maybe kvmclock should be moved to planex.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  arch/x86/include/asm/kvm_host.h | 3 +++
>  arch/x86/kvm/x86.c              | 6 ++++++
>  2 files changed, 9 insertions(+)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 0344e8bed319..d0cb177b6f52 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -2339,6 +2339,8 @@ enum {
>  # define kvm_memslots_for_spte_role(kvm, role) __kvm_memslots(kvm, 0)
>  #endif
>  
> +#define KVM_MAX_VCPU_PLANES	16

I'm pretty sure x86 can't support 16 planes.  "union kvm_mmu_page_role" needs
to incorporate the plane, otherwise per-plane memory attributes will won't work.
And adding four bits to the plane would theoretically put us in danger of
overflowing gfn_write_track (in practice, I highly, highly doubt that can happen).

Why not start with 4 planes?  Or even 2?  Expanding the number of planes should
be much easier than contracting.  Based on the VTL and VMPL roadmaps, 4 planes
will probably be enough for many years to come.