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>

1 files changed, 25 insertions, 0 deletions

diff --git a/drivers/kvm/x86.c b/drivers/kvm/x86.c
index ac09f381f47f..15e1203faef0 100644
--- a/drivers/kvm/x86.c
+++ b/drivers/kvm/x86.c
@@ -166,6 +166,26 @@ out:
         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");