Re: [PATCH v3 2/3] KVM: x86/mmu: Avoid collision with !PRESENT SPTEs in TDP MMU lpage stats

[David Matlack]

On Fri, Jul 30, 2021 at 03:59:38PM -0700, Mingwei Zhang wrote:
> From: Sean Christopherson <seanjc@xxxxxxxxxx>
> 
> Factor in whether or not the old/new SPTEs are shadow-present when
> adjusting the large page stats in the TDP MMU.  A modified MMIO SPTE can
> toggle the page size bit, as bit 7 is used to store the MMIO generation,
> i.e. is_large_pte() can get a false positive when called on a MMIO SPTE.
> Ditto for nuking SPTEs with REMOVED_SPTE, which sets bit 7 in its magic
> value.
> 
> Opportunistically move the logic below the check to verify at least one
> of the old/new SPTEs is shadow present.
> 
> Use is/was_leaf even though is/was_present would suffice.  The code
> generation is roughly equivalent since all flags need to be computed
> prior to the code in question, and using the *_leaf flags will minimize
> the diff in a future enhancement to account all pages, i.e. will change
> the check to "is_leaf != was_leaf".
> 
> Reviewed-by: Ben Gardon <bgardon@xxxxxxxxxx>
> 
> Fixes: 1699f65c8b65 ("kvm/x86: Fix 'lpages' kvm stat for TDM MMU")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
> ---

Reviewed-by: David Matlack <dmatlack@xxxxxxxxxx>

>  arch/x86/kvm/mmu/tdp_mmu.c | 20 +++++++++++++-------
>  1 file changed, 13 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index caac4ddb46df..cba2ab5db2a0 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -413,6 +413,7 @@ static void __handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
>  	bool was_leaf = was_present && is_last_spte(old_spte, level);
>  	bool is_leaf = is_present && is_last_spte(new_spte, level);
>  	bool pfn_changed = spte_to_pfn(old_spte) != spte_to_pfn(new_spte);
> +	bool was_large, is_large;
>  
>  	WARN_ON(level > PT64_ROOT_MAX_LEVEL);
>  	WARN_ON(level < PG_LEVEL_4K);
> @@ -446,13 +447,6 @@ static void __handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
>  
>  	trace_kvm_tdp_mmu_spte_changed(as_id, gfn, level, old_spte, new_spte);
>  
> -	if (is_large_pte(old_spte) != is_large_pte(new_spte)) {
> -		if (is_large_pte(old_spte))
> -			atomic64_sub(1, (atomic64_t*)&kvm->stat.lpages);
> -		else
> -			atomic64_add(1, (atomic64_t*)&kvm->stat.lpages);
> -	}
> -
>  	/*
>  	 * The only times a SPTE should be changed from a non-present to
>  	 * non-present state is when an MMIO entry is installed/modified/
> @@ -478,6 +472,18 @@ static void __handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
>  		return;
>  	}
>  
> +	/*
> +	 * Update large page stats if a large page is being zapped, created, or
> +	 * is replacing an existing shadow page.
> +	 */
> +	was_large = was_leaf && is_large_pte(old_spte);
> +	is_large = is_leaf && is_large_pte(new_spte);
> +	if (was_large != is_large) {
> +		if (was_large)
> +			atomic64_sub(1, (atomic64_t *)&kvm->stat.lpages);
> +		else
> +			atomic64_add(1, (atomic64_t *)&kvm->stat.lpages);
> +	}
>  
>  	if (was_leaf && is_dirty_spte(old_spte) &&
>  	    (!is_present || !is_dirty_spte(new_spte) || pfn_changed))
> -- 
> 2.32.0.554.ge1b32706d8-goog
>