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

[Yan Zhao]

On Wed, Jun 19, 2024 at 03:36:10PM -0700, Rick Edgecombe wrote:
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index e9c1783a8743..287dcc2685e4 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3701,7 +3701,9 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu *vcpu)
>  	int r;
>  
>  	if (tdp_mmu_enabled) {
> -		kvm_tdp_mmu_alloc_root(vcpu);
> +		if (kvm_has_mirrored_tdp(vcpu->kvm))
> +			kvm_tdp_mmu_alloc_root(vcpu, true);
> +		kvm_tdp_mmu_alloc_root(vcpu, false);
>  		return 0;
>  	}
mmu_alloc_direct_roots() is called when vcpu->arch.mmu->root.hpa is INVALID_PAGE
in kvm_mmu_reload(), which could happen after direct root is invalidated.

> -void kvm_tdp_mmu_alloc_root(struct kvm_vcpu *vcpu)
> +void kvm_tdp_mmu_alloc_root(struct kvm_vcpu *vcpu, bool mirror)
>  {
>  	struct kvm_mmu *mmu = vcpu->arch.mmu;
>  	union kvm_mmu_page_role role = mmu->root_role;
> @@ -241,6 +246,9 @@ void kvm_tdp_mmu_alloc_root(struct kvm_vcpu *vcpu)
>  	struct kvm *kvm = vcpu->kvm;
>  	struct kvm_mmu_page *root;
>  
> +	if (mirror)
> +		role.is_mirror = 1;
> +
Could we add a validity check of mirror_root_hpa to prevent an incorrect ref
count increment of the mirror root?

+       if (mirror) {
+               if (mmu->mirror_root_hpa != INVALID_PAGE)
+                       return;
+
                role.is_mirror = true;
+       }

>  	/*
>  	 * Check for an existing root before acquiring the pages lock to avoid
>  	 * unnecessary serialization if multiple vCPUs are loading a new root.
> @@ -292,8 +300,12 @@ void kvm_tdp_mmu_alloc_root(struct kvm_vcpu *vcpu)
>  	 * and actually consuming the root if it's invalidated after dropping
>  	 * mmu_lock, and the root can't be freed as this vCPU holds a reference.
>  	 */
> -	mmu->root.hpa = __pa(root->spt);
> -	mmu->root.pgd = 0;
> +	if (mirror) {
> +		mmu->mirror_root_hpa = __pa(root->spt);
> +	} else {
> +		mmu->root.hpa = __pa(root->spt);
> +		mmu->root.pgd = 0;
> +	}
>  }