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

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...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>
This commit is contained in:
Sean Christopherson 2019-02-05 13:01:32 -08:00 committed by Paolo Bonzini
parent 7390de1e99
commit 8a674adc11
1 changed files with 4 additions and 1 deletions
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5
arch/x86/kvm/mmu.c
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@ -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);