linux-kernel - Re: [PATCH 3/4] KVM: x86/mmu: Use raw level to index into MMIO walks' sptes array

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Date:   Fri, 18 Dec 2020 10:18:43 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Ben Gardon <bgardon@...gle.com>,
        Richard Herbert <rherbert@...patico.ca>
Subject: Re: [PATCH 3/4] KVM: x86/mmu: Use raw level to index into MMIO
 walks' sptes array

Sean Christopherson <seanjc@...gle.com> writes:

> Bump the size of the sptes array by one and use the raw level of the
> SPTE to index into the sptes array.  Using the SPTE level directly
> improves readability by eliminating the need to reason out why the level
> is being adjusted when indexing the array.  The array is on the stack
> and is not explicitly initialized; bumping its size is nothing more than
> a superficial adjustment to the stack frame.
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/mmu/mmu.c     | 15 +++++++--------
>  arch/x86/kvm/mmu/tdp_mmu.c |  2 +-
>  2 files changed, 8 insertions(+), 9 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 52f36c879086..4798a4472066 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3500,7 +3500,7 @@ static int get_walk(struct kvm_vcpu *vcpu, u64 addr, u64 *sptes, int *root_level
>  		leaf = iterator.level;
>  		spte = mmu_spte_get_lockless(iterator.sptep);
>  
> -		sptes[leaf - 1] = spte;
> +		sptes[leaf] = spte;
>  
>  		if (!is_shadow_present_pte(spte))
>  			break;
> @@ -3514,7 +3514,7 @@ static int get_walk(struct kvm_vcpu *vcpu, u64 addr, u64 *sptes, int *root_level
>  /* return true if reserved bit is detected on spte. */
>  static bool get_mmio_spte(struct kvm_vcpu *vcpu, u64 addr, u64 *sptep)
>  {
> -	u64 sptes[PT64_ROOT_MAX_LEVEL];
> +	u64 sptes[PT64_ROOT_MAX_LEVEL + 1];
>  	struct rsvd_bits_validate *rsvd_check;
>  	int root, leaf, level;
>  	bool reserved = false;
> @@ -3537,16 +3537,15 @@ static bool get_mmio_spte(struct kvm_vcpu *vcpu, u64 addr, u64 *sptep)
>  	rsvd_check = &vcpu->arch.mmu->shadow_zero_check;
>  
>  	for (level = root; level >= leaf; level--) {
> -		if (!is_shadow_present_pte(sptes[level - 1]))
> +		if (!is_shadow_present_pte(sptes[level]))
>  			break;
>  		/*
>  		 * Use a bitwise-OR instead of a logical-OR to aggregate the
>  		 * reserved bit and EPT's invalid memtype/XWR checks to avoid
>  		 * adding a Jcc in the loop.
>  		 */
> -		reserved |= __is_bad_mt_xwr(rsvd_check, sptes[level - 1]) |
> -			    __is_rsvd_bits_set(rsvd_check, sptes[level - 1],
> -					       level);
> +		reserved |= __is_bad_mt_xwr(rsvd_check, sptes[level]) |
> +			    __is_rsvd_bits_set(rsvd_check, sptes[level], level);
>  	}
>  
>  	if (reserved) {
> @@ -3554,10 +3553,10 @@ static bool get_mmio_spte(struct kvm_vcpu *vcpu, u64 addr, u64 *sptep)
>  		       __func__, addr);
>  		for (level = root; level >= leaf; level--)
>  			pr_err("------ spte 0x%llx level %d.\n",
> -			       sptes[level - 1], level);
> +			       sptes[level], level);
>  	}
>  
> -	*sptep = sptes[leaf - 1];
> +	*sptep = sptes[leaf];
>  
>  	return reserved;
>  }
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index a4f9447f8327..efef571806ad 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -1160,7 +1160,7 @@ int kvm_tdp_mmu_get_walk(struct kvm_vcpu *vcpu, u64 addr, u64 *sptes,
>  
>  	tdp_mmu_for_each_pte(iter, mmu, gfn, gfn + 1) {
>  		leaf = iter.level;
> -		sptes[leaf - 1] = iter.old_spte;
> +		sptes[leaf] = iter.old_spte;
>  	}
>  
>  	return leaf;

An alretnitive solution would've been to reverse the array and fill it
like

 sptes[PT64_ROOT_MAX_LEVEL - leaf] = iter.old_spte;

but this may not reach the goal of 'improved readability' :-)

Also, we may add an MMU_DEBUG ifdef-ed check that sptes[0] remains
intact.

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly