KVM: Do not report TPR write to userspace if new value bigger or equal to a previous one.

Saves many exits to userspace in a case of IRQ chip in userspace.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

2 files changed, 16 insertions, 7 deletions

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 143818eff52e..e283a63b2bca 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1862,9 +1862,13 @@ static int emulate_on_interception(struct vcpu_svm *svm,
 
 static int cr8_write_interception(struct vcpu_svm *svm, struct kvm_run *kvm_run)
 {
+        u8 cr8_prev = kvm_get_cr8(&svm->vcpu);
+        /* instruction emulation calls kvm_set_cr8() */
         emulate_instruction(&svm->vcpu, NULL, 0, 0, 0);
         if (irqchip_in_kernel(svm->vcpu.kvm))
                 return 1;
+        if (cr8_prev <= kvm_get_cr8(&svm->vcpu))
+                return 1;
         kvm_run->exit_reason = KVM_EXIT_SET_TPR;
         return 0;
 }
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 9162b4cd3410..51f804c8fe79 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -2724,13 +2724,18 @@ static int handle_cr(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
                         kvm_set_cr4(vcpu, kvm_register_read(vcpu, reg));
                         skip_emulated_instruction(vcpu);
                         return 1;
-                case 8:
-                        kvm_set_cr8(vcpu, kvm_register_read(vcpu, reg));
-                        skip_emulated_instruction(vcpu);
-                        if (irqchip_in_kernel(vcpu->kvm))
-                                return 1;
-                        kvm_run->exit_reason = KVM_EXIT_SET_TPR;
-                        return 0;
+                case 8: {
+                                u8 cr8_prev = kvm_get_cr8(vcpu);
+                                u8 cr8 = kvm_register_read(vcpu, reg);
+                                kvm_set_cr8(vcpu, cr8);
+                                skip_emulated_instruction(vcpu);
+                                if (irqchip_in_kernel(vcpu->kvm))
+                                        return 1;
+                                if (cr8_prev <= cr8)
+                                        return 1;
+                                kvm_run->exit_reason = KVM_EXIT_SET_TPR;
+                                return 0;
+                        }
                 };
                 break;
         case 2: /* clts */