KVM: Don't bother the mmu if cr3 load doesn't change cr3

If the guest requests just a tlb flush, don't take the vm lock and
drop the mmu context pointlessly.
Signed-off-by: Avi Kivity <avi@qumranet.com>

drivers/kvm/mmu.c

@@ -1086,7 +1086,7 @@ static int nonpaging_init_context(struct kvm_vcpu *vcpu)
 	return 0;
 }
 
-static void kvm_mmu_flush_tlb(struct kvm_vcpu *vcpu)
+void kvm_mmu_flush_tlb(struct kvm_vcpu *vcpu)
 {
 	++vcpu->stat.tlb_flush;
 	kvm_x86_ops->tlb_flush(vcpu);

drivers/kvm/x86.c

@@ -166,6 +166,26 @@ int load_pdptrs(struct kvm_vcpu *vcpu, unsigned long cr3)
 	return ret;
 }
 
+static bool pdptrs_changed(struct kvm_vcpu *vcpu)
+{
+	u64 pdpte[ARRAY_SIZE(vcpu->pdptrs)];
+	bool changed = true;
+	int r;
+
+	if (is_long_mode(vcpu) || !is_pae(vcpu))
+		return false;
+
+	mutex_lock(&vcpu->kvm->lock);
+	r = kvm_read_guest(vcpu->kvm, vcpu->cr3 & ~31u, pdpte, sizeof(pdpte));
+	if (r < 0)
+		goto out;
+	changed = memcmp(pdpte, vcpu->pdptrs, sizeof(pdpte)) != 0;
+out:
+	mutex_unlock(&vcpu->kvm->lock);
+
+	return changed;
+}
+
 void set_cr0(struct kvm_vcpu *vcpu, unsigned long cr0)
 {
 	if (cr0 & CR0_RESERVED_BITS) {
@@ -271,6 +291,11 @@ EXPORT_SYMBOL_GPL(set_cr4);
 
 void set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
 {
+	if (cr3 == vcpu->cr3 && !pdptrs_changed(vcpu)) {
+		kvm_mmu_flush_tlb(vcpu);
+		return;
+	}
+
 	if (is_long_mode(vcpu)) {
 		if (cr3 & CR3_L_MODE_RESERVED_BITS) {
 			printk(KERN_DEBUG "set_cr3: #GP, reserved bits\n");

drivers/kvm/x86.h

@@ -299,6 +299,7 @@ int emulator_write_emulated(unsigned long addr,
 
 unsigned long segment_base(u16 selector);
 
+void kvm_mmu_flush_tlb(struct kvm_vcpu *vcpu);
 void kvm_mmu_pte_write(struct kvm_vcpu *vcpu, gpa_t gpa,
 		       const u8 *new, int bytes);
 int kvm_mmu_unprotect_page_virt(struct kvm_vcpu *vcpu, gva_t gva);