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

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Message-ID: <d33d00b88707961126a24b19f940de43ba6e6c56.camel@intel.com>
Date: Tue, 9 Jul 2024 22:36:55 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "Zhao, Yan Y" <yan.y.zhao@...el.com>
CC: "pbonzini@...hat.com" <pbonzini@...hat.com>, "seanjc@...gle.com"
	<seanjc@...gle.com>, "Huang, Kai" <kai.huang@...el.com>, "sagis@...gle.com"
	<sagis@...gle.com>, "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
	"Aktas, Erdem" <erdemaktas@...gle.com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "dmatlack@...gle.com" <dmatlack@...gle.com>,
	"Yamahata, Isaku" <isaku.yamahata@...el.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 13/17] KVM: x86/tdp_mmu: Support mirror root for TDP
 MMU

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;