linux-kernel - Re: [RFC PATCH v5 054/104] KVM: x86/tdp_mmu: Keep PRIVATE

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Date:   Thu, 07 Apr 2022 13:43:11 +1200
From:   Kai Huang <kai.huang@...el.com>
To:     isaku.yamahata@...el.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     isaku.yamahata@...il.com, Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>, erdemaktas@...gle.com,
        Connor Kuehl <ckuehl@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [RFC PATCH v5 054/104] KVM: x86/tdp_mmu: Keep PRIVATE_PROHIBIT
 bit when zapping

On Fri, 2022-03-04 at 11:49 -0800, isaku.yamahata@...el.com wrote:
> From: Isaku Yamahata <isaku.yamahata@...el.com>
> 
> SPTE_PRIVATE_PROHIBIT specifies the share or private GPA is allowed or not.
> It needs to be kept over zapping the EPT entry.  Currently the EPT entry is
> initialized shadow_init_value unconditionally to clear
> SPTE_PRIVATE_PROHIBIT bit.  To carry SPTE_PRIVATE_PROHIBIT bit, introduce a
> helper function to get initial value for zapped entry with
> SPTE_PRIVATE_PROHIBIT bit.  Replace shadow_init_value with it.

Isn't it better to merge patch 53-55, especially 54-55 together? 

> 
> Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 19 +++++++++++++++----
>  1 file changed, 15 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 1949f81027a0..6d750563824d 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -610,6 +610,12 @@ static inline bool tdp_mmu_set_spte_atomic(struct kvm *kvm,
>  	return true;
>  }
>  
> +static u64 shadow_init_spte(u64 old_spte)
> +{
> +	return shadow_init_value |
> +		(is_private_prohibit_spte(old_spte) ? SPTE_PRIVATE_PROHIBIT : 0);
> +}
> +
>  static inline bool tdp_mmu_zap_spte_atomic(struct kvm *kvm,
>  					   struct tdp_iter *iter)
>  {
> @@ -641,7 +647,8 @@ static inline bool tdp_mmu_zap_spte_atomic(struct kvm *kvm,
>  	 * shadow_init_value (which sets "suppress #VE" bit) so it
>  	 * can be set when EPT table entries are zapped.
>  	 */
> -	WRITE_ONCE(*rcu_dereference(iter->sptep), shadow_init_value);
> +	WRITE_ONCE(*rcu_dereference(iter->sptep),
> +		shadow_init_spte(iter->old_spte));
>  
>  	return true;
>  }

In this and next patch (54-55), in all the code path, you already have the iter-
>sptep, from which you can get the sp->private_sp, and check using
is_private_sp().  Why do we need this SPTE_PRIVATE_PRORHIBIT bit?

Are you suggesting we can have mixed private/shared mapping under a private_sp?

> @@ -853,7 +860,8 @@ static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
>  
>  		if (!shared) {
>  			/* see comments in tdp_mmu_zap_spte_atomic() */
> -			tdp_mmu_set_spte(kvm, &iter, shadow_init_value);
> +			tdp_mmu_set_spte(kvm, &iter,
> +					shadow_init_spte(iter.old_spte));
>  			flush = true;
>  		} else if (!tdp_mmu_zap_spte_atomic(kvm, &iter)) {
>  			/*
> @@ -1038,11 +1046,14 @@ static int tdp_mmu_map_handle_target_level(struct kvm_vcpu *vcpu,
>  		new_spte = make_mmio_spte(vcpu,
>  				tdp_iter_gfn_unalias(vcpu->kvm, iter),
>  				pte_access);
> -	else
> +	else {
>  		wrprot = make_spte(vcpu, sp, fault->slot, pte_access,
>  				tdp_iter_gfn_unalias(vcpu->kvm, iter),
>  				fault->pfn, iter->old_spte, fault->prefetch,
>  				true, fault->map_writable, &new_spte);
> +		if (is_private_prohibit_spte(iter->old_spte))
> +			new_spte |= SPTE_PRIVATE_PROHIBIT;
> +	}
>  
>  	if (new_spte == iter->old_spte)
>  		ret = RET_PF_SPURIOUS;
> @@ -1335,7 +1346,7 @@ static bool set_spte_gfn(struct kvm *kvm, struct tdp_iter *iter,
>  	 * invariant that the PFN of a present * leaf SPTE can never change.
>  	 * See __handle_changed_spte().
>  	 */
> -	tdp_mmu_set_spte(kvm, iter, shadow_init_value);
> +	tdp_mmu_set_spte(kvm, iter, shadow_init_spte(iter->old_spte));
>  
>  	if (!pte_write(range->pte)) {
>  		new_spte = kvm_mmu_changed_pte_notifier_make_spte(iter->old_spte,