[PATCH v3] KVM: x86/MMU: Do not check unsync status for root SP.

[Yu Zhang <yu.c.zhang@linux.intel.com>]

In shadow page table, only leaf SPs may be marked as unsync;
instead, for non-leaf SPs, we store the number of unsynced
children in unsync_children. Therefore, in kvm_mmu_sync_root(),
sp->unsync shall always be zero for the root SP and there is
no need to check it. Remove the check, and add a warning
inside mmu_sync_children() to assert that the flags are used
properly.

While at it, move the warning from mmu_need_write_protect()
to kvm_unsync_page().

Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
---
 arch/x86/kvm/mmu/mmu.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 86af58294272..5f482af125b4 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -1995,6 +1995,12 @@ static void mmu_sync_children(struct kvm_vcpu *vcpu,
 	LIST_HEAD(invalid_list);
 	bool flush = false;
 
+	/*
+	 * Only 4k SPTEs can directly be made unsync, the parent pages
+	 * should never be unsyc'd.
+	 */
+	WARN_ON_ONCE(parent->unsync);
+
 	while (mmu_unsync_walk(parent, &pages)) {
 		bool protected = false;
 
@@ -2502,6 +2508,8 @@ EXPORT_SYMBOL_GPL(kvm_mmu_unprotect_page);
 
 static void kvm_unsync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
 {
+	WARN_ON(sp->role.level != PG_LEVEL_4K);
+
 	trace_kvm_mmu_unsync_page(sp);
 	++vcpu->kvm->stat.mmu_unsync;
 	sp->unsync = 1;
@@ -2524,7 +2532,6 @@ bool mmu_need_write_protect(struct kvm_vcpu *vcpu, gfn_t gfn,
 		if (sp->unsync)
 			continue;
 
-		WARN_ON(sp->role.level != PG_LEVEL_4K);
 		kvm_unsync_page(vcpu, sp);
 	}
 
@@ -3406,8 +3413,7 @@ void kvm_mmu_sync_roots(struct kvm_vcpu *vcpu)
 		 * mmu_need_write_protect() describe what could go wrong if this
 		 * requirement isn't satisfied.
 		 */
-		if (!smp_load_acquire(&sp->unsync) &&
-		    !smp_load_acquire(&sp->unsync_children))
+		if (!smp_load_acquire(&sp->unsync_children))
 			return;
 
 		write_lock(&vcpu->kvm->mmu_lock);
-- 
2.17.1