[PATCH v2] KVM: x86/MMU: Do not check unsync status for root SP.

[Yu Zhang]

In shadow page table, only leaf SPs may be marked as unsync.
And for non-leaf SPs, we use unsync_children to keep the number
of the unsynced children. In kvm_mmu_sync_root(), sp->unsync
shall always be zero for the root SP, , hence no need to check
it. Instead, a warning inside mmu_sync_children() is added, in
case someone incorrectly used it.

Also, clarify the mmu_need_write_protect(), by moving the warning
into kvm_unsync_page().

Signed-off-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
Changes in V2:
- warnings added based on Sean's suggestion.

 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 86af582..c4797a00cc 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(sp->unsync);
+
 	while (mmu_unsync_walk(parent, &pages)) {
 		bool protected = false;
 
@@ -2502,6 +2508,8 @@ int kvm_mmu_unprotect_page(struct kvm *kvm, gfn_t gfn)
 
 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);
-- 
1.9.1