linux-kernel - Re: [PATCH v2 25/28] KVM: x86/mmu: Allow zapping collapsible SPTEs to use MMU read lock

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Message-ID: <e87d4a5d-f6ac-677a-87aa-0c30977c92f1@redhat.com>
Date:   Wed, 3 Feb 2021 12:34:24 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Ben Gardon <bgardon@...gle.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Peter Xu <peterx@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        Peter Feiner <pfeiner@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH v2 25/28] KVM: x86/mmu: Allow zapping collapsible SPTEs to
 use MMU read lock

On 02/02/21 19:57, Ben Gardon wrote:
> @@ -1485,7 +1489,9 @@ void kvm_tdp_mmu_zap_collapsible_sptes(struct kvm *kvm,
>  	struct kvm_mmu_page *root;
>  	int root_as_id;
>  
> -	for_each_tdp_mmu_root_yield_safe(kvm, root, false) {
> +	read_lock(&kvm->mmu_lock);
> +
> +	for_each_tdp_mmu_root_yield_safe(kvm, root, true) {
>  		root_as_id = kvm_mmu_page_as_id(root);
>  		if (root_as_id != slot->as_id)
>  			continue;
> @@ -1493,6 +1499,8 @@ void kvm_tdp_mmu_zap_collapsible_sptes(struct kvm *kvm,
>  		zap_collapsible_spte_range(kvm, root, slot->base_gfn,
>  					   slot->base_gfn + slot->npages);
>  	}
> +
> +	read_unlock(&kvm->mmu_lock);
>  }


I'd prefer the functions to be consistent about who takes the lock, 
either mmu.c or tdp_mmu.c.  Since everywhere else you're doing it in 
mmu.c, that would be:

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 0554d9c5c5d4..386ee4b703d9 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -5567,10 +5567,13 @@ void kvm_mmu_zap_collapsible_sptes(struct kvm *kvm,
  	write_lock(&kvm->mmu_lock);
  	slot_handle_leaf(kvm, (struct kvm_memory_slot *)memslot,
  			 kvm_mmu_zap_collapsible_spte, true);
+	write_unlock(&kvm->mmu_lock);

-	if (kvm->arch.tdp_mmu_enabled)
+	if (kvm->arch.tdp_mmu_enabled) {
+		read_lock(&kvm->mmu_lock);
  		kvm_tdp_mmu_zap_collapsible_sptes(kvm, memslot);
-	write_unlock(&kvm->mmu_lock);
+		read_unlock(&kvm->mmu_lock);
+	}
  }

  void kvm_arch_flush_remote_tlbs_memslot(struct kvm *kvm,

and just lockdep_assert_held_read here.

> -		tdp_mmu_set_spte(kvm, &iter, 0);
> -
> -		spte_set = true;

Is it correct to remove this assignment?

Paolo