KVM: x86/mmu: skip over invalid root pages when zapping all sptes

...to guarantee forward progress.  When zapped, root pages are marked
invalid and moved to the head of the active pages list until they are
explicitly freed.  Theoretically, having unzappable root pages at the
head of the list could prevent kvm_mmu_zap_all() from making forward
progress were a future patch to add a loop restart after processing a
page, e.g. to drop mmu_lock on contention.

Although kvm_mmu_prepare_zap_page() can theoretically take action on
invalid pages, e.g. to zap unsync children, functionally it's not
necessary (root pages will be re-zapped when freed) and practically
speaking the odds of e.g. @unsync or @unsync_children becoming %true
while zapping all pages is basically nil.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 5cdeb88850f8f24a98867f3ce5abab00b8c37d1d..c79ad7f31fdb20487c31aa5ac216cd6ad57c24bc 100644 (file)
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -5853,9 +5853,12 @@ void kvm_mmu_zap_all(struct kvm *kvm)
 
        spin_lock(&kvm->mmu_lock);
 restart:
-       list_for_each_entry_safe(sp, node, &kvm->arch.active_mmu_pages, link)
+       list_for_each_entry_safe(sp, node, &kvm->arch.active_mmu_pages, link) {
+               if (sp->role.invalid && sp->root_count)
+                       continue;
                if (kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list))
                        goto restart;
+       }
 
        kvm_mmu_commit_zap_page(kvm, &invalid_list);
        spin_unlock(&kvm->mmu_lock);