Re: [PATCH v3 13/17] KVM: x86/tdp_mmu: Support mirror root for TDP MMU

[Edgecombe, Rick P]

On Thu, 2024-07-04 at 16:09 +0800, Yan Zhao wrote:
> Perhaps also a comment in kvm_mmu_reload() to address concerns like why
> checking
> only root.hpa in kvm_mmu_reload() is enough.

Sounds good, and thanks again for catching this.

> 
> diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> index 03737f3aaeeb..aba98c8cc67d 100644
> --- a/arch/x86/kvm/mmu.h
> +++ b/arch/x86/kvm/mmu.h
> @@ -129,6 +129,15 @@ void kvm_mmu_track_write(struct kvm_vcpu *vcpu, gpa_t
> gpa, const u8 *new,
>  
>  static inline int kvm_mmu_reload(struct kvm_vcpu *vcpu)
>  {
> +       /*
> +        * Checking root.hpa is sufficient even when KVM has mirror root.
> +        * We can have either:
> +        * (1) mirror_root_hpa = INVALID_PAGE, root.hpa = INVALID_PAGE
> +        * (2) mirror_root_hpa = root ,        root.hpa = INVALID_PAGE
Looks good to me except for the space  ^

> +        * (3) mirror_root_hpa = root1,        root.hpa = root2
> +        * We don't ever have:
> +        *     mirror_root_hpa = INVALID_PAGE, root.hpa = root
> +        */
>         if (likely(vcpu->arch.mmu->root.hpa != INVALID_PAGE))
>                 return 0;
>  
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index a5f803f1d17e..eee35e958971 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3705,7 +3705,8 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu *vcpu)
>         int r;
>  
>         if (tdp_mmu_enabled) {
> -               if (kvm_has_mirrored_tdp(vcpu->kvm))
> +               if (kvm_has_mirrored_tdp(vcpu->kvm) &&
> +                   !VALID_PAGE(mmu->mirror_root_hpa))
>                         kvm_tdp_mmu_alloc_root(vcpu, true);
>                 kvm_tdp_mmu_alloc_root(vcpu, false);
>                 return 0;