[PATCH 5/5] KVM: VMX: Handle RFLAGS.TF in skip_emulated_instruction

[Kyle Huey <me@kylehuey.com>]

Similar to the code in kvm_vcpu_check_singlestep, check for TF and,
depending on the origin, synthesize a DB exception or an exit to userspace.

Signed-off-by: Kyle Huey <khuey@kylehuey.com>
---
 arch/x86/kvm/vmx.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index f404aef..6583e97 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -2470,16 +2470,37 @@ static void skip_emulated_instruction_no_trap(struct kvm_vcpu *vcpu)
 
 	/* skipping an emulated instruction also counts */
 	vmx_set_interrupt_shadow(vcpu, 0);
 }
 
 static int skip_emulated_instruction(struct kvm_vcpu *vcpu)
 {
 	skip_emulated_instruction_no_trap(vcpu);
+
+	if (unlikely(vmx_get_rflags(vcpu) & X86_EFLAGS_TF)) {
+		if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
+			vcpu->run->debug.arch.dr6 = DR6_BS | DR6_FIXED_1 |
+						    DR6_RTM;
+			vcpu->run->debug.arch.pc = vcpu->arch.singlestep_rip;
+			vcpu->run->debug.arch.exception = DB_VECTOR;
+			vcpu->run->exit_reason = KVM_EXIT_DEBUG;
+			return 0;
+		}
+
+		/*
+		 * "Certain debug exceptions may clear bit 0-3.  The
+		 * remaining contents of the DR6 register are never
+		 * cleared by the processor".
+		 */
+		vcpu->arch.dr6 &= ~15;
+		vcpu->arch.dr6 |= DR6_BS | DR6_RTM;
+		kvm_queue_exception(vcpu, DB_VECTOR);
+	}
+
 	return 1;
 }
 
 /*
  * KVM wants to inject page-faults which it got to the guest. This function
  * checks whether in a nested guest, we need to inject them to L1 or L2.
  */
 static int nested_vmx_check_exception(struct kvm_vcpu *vcpu, unsigned nr)
-- 
2.10.2