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

[Paolo Bonzini]

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