Re: [PATCH v1 1/3] KVM: x86: Convert TDP level calculation to vendor's specific code

[Wei Huang <wei.huang2@xxxxxxx>]

On 8/5/21 4:51 PM, Sean Christopherson wrote:
> If we want to keep the MAXPHYADDR behavior, I'd vote for something like:
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index b4b65c21b2ca..7e35f2bf89b4 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -97,6 +97,7 @@ module_param_named(flush_on_reuse, force_flush_and_sync_on_reuse, bool, 0644);
>   bool tdp_enabled = false;
>
>   static int max_huge_page_level __read_mostly;
> +static int tdp_root_level __read_mostly;
>   static int max_tdp_level __read_mostly;
>
>   enum {
> @@ -4645,6 +4646,9 @@ static union kvm_mmu_role kvm_calc_mmu_role_common(struct kvm_vcpu *vcpu,
>
>   static inline int kvm_mmu_get_tdp_level(struct kvm_vcpu *vcpu)
>   {
> +       if (tdp_root_level)
> +               return tdp_root_level;
> +
>          /* Use 5-level TDP if and only if it's useful/necessary. */
>          if (max_tdp_level == 5 && cpuid_maxphyaddr(vcpu) <= 48)
>                  return 4;
> @@ -5336,10 +5340,11 @@ void kvm_mmu_invpcid_gva(struct kvm_vcpu *vcpu, gva_t gva, unsigned long pcid)
>           */
>   }
>
> -void kvm_configure_mmu(bool enable_tdp, int tdp_max_root_level,
> -                      int tdp_huge_page_level)
> +void kvm_configure_mmu(bool enable_tdp, int tdp_forced_root_level,
> +                      int tdp_max_root_level, int tdp_huge_page_level)
>   {
>          tdp_enabled = enable_tdp;
> +       tdp_root_level = tdp_forced_root_level;
>          max_tdp_level = tdp_max_root_level;
>
>          /*

I decided to take this suggestion in v2: it avoids using 5-level table 
(memory cost and potential table-walk overhead) if the host has the 
flexibility of using 4-level NPT under LA57.