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