linux-kernel - Re: [PATCH v2 2/3] KVM: x86: Handle the case of 5-level shadow page table

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Date:   Mon, 9 Aug 2021 15:17:12 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Wei Huang <wei.huang2@....com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, vkuznets@...hat.com, wanpengli@...cent.com,
        jmattson@...gle.com, joro@...tes.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, x86@...nel.org, hpa@...or.com
Subject: Re: [PATCH v2 2/3] KVM: x86: Handle the case of 5-level shadow page
 table

On Sun, Aug 08, 2021, Wei Huang wrote:
> @@ -3457,10 +3457,19 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
>  		mmu->pae_root[i] = root | pm_mask;
>  	}
>  
> -	if (mmu->shadow_root_level == PT64_ROOT_4LEVEL)
> +	/*
> +	 * Depending on the shadow_root_level, build the root_hpa table by
> +	 * chaining either pml5->pml4->pae or pml4->pae.
> +	 */
> +	mmu->root_hpa = __pa(mmu->pae_root);
> +	if (mmu->shadow_root_level >= PT64_ROOT_4LEVEL) {
> +		mmu->pml4_root[0] = mmu->root_hpa | pm_mask;
>  		mmu->root_hpa = __pa(mmu->pml4_root);
> -	else
> -		mmu->root_hpa = __pa(mmu->pae_root);
> +	}
> +	if (mmu->shadow_root_level == PT64_ROOT_5LEVEL) {
> +		mmu->pml5_root[0] = mmu->root_hpa | pm_mask;
> +		mmu->root_hpa = __pa(mmu->pml5_root);
> +	}

I still really dislike this approach, it requires visually connecting multiple
statements to understand the chain.  I don't see any advantage (the 6-level paging
comment was 99.9% a joke) of rewriting root_hpa other than that's how it's done today.

In the future, please give reviewers ample opportunity to respond before sending
a new version if there's disagreement, otherwise the conversation gets carried
over into a different thread and loses the original context.

>  
>  set_root_pgd:
>  	mmu->root_pgd = root_pgd;