KVM: x86: handle singlestep during emulation

This lets debugging work better during emulation of invalid
guest state.

This time the check is done after emulation, but before writeback
of the flags; we need to check the flags *before* execution of the
instruction, we cannot check singlestep_rip because the CS base may
have already been modified.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Conflicts:
	arch/x86/kvm/x86.c

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 8931318..05afd1a 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4971,6 +4971,41 @@ static int kvm_vcpu_check_hw_bp(unsigned long addr, u32 type, u32 dr7,
        return dr6;
 }
 
+static void kvm_vcpu_check_singlestep(struct kvm_vcpu *vcpu, int *r)
+{
+       struct kvm_run *kvm_run = vcpu->run;
+
+       /*
+        * Use the "raw" value to see if TF was passed to the processor.
+        * Note that the new value of the flags has not been saved yet.
+        *
+        * This is correct even for TF set by the guest, because "the
+        * processor will not generate this exception after the instruction
+        * that sets the TF flag".
+        */
+       unsigned long rflags = kvm_x86_ops->get_rflags(vcpu);
+
+       if (unlikely(rflags & X86_EFLAGS_TF)) {
+               if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
+                       kvm_run->debug.arch.dr6 = DR6_BS | DR6_FIXED_1;
+                       kvm_run->debug.arch.pc = vcpu->arch.singlestep_rip;
+                       kvm_run->debug.arch.exception = DB_VECTOR;
+                       kvm_run->exit_reason = KVM_EXIT_DEBUG;
+                       *r = EMULATE_USER_EXIT;
+               } else {
+                       vcpu->arch.emulate_ctxt.eflags &= ~X86_EFLAGS_TF;
+                       /*
+                        * "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;
+                       kvm_queue_exception(vcpu, DB_VECTOR);
+               }
+       }
+}
+
 static bool kvm_vcpu_check_breakpoint(struct kvm_vcpu *vcpu, int *r)
 {
        struct kvm_run *kvm_run = vcpu->run;
@@ -5117,10 +5152,12 @@ restart:
 
        if (writeback) {
                toggle_interruptibility(vcpu, ctxt->interruptibility);
-               kvm_set_rflags(vcpu, ctxt->eflags);
                kvm_make_request(KVM_REQ_EVENT, vcpu);
                vcpu->arch.emulate_regs_need_sync_to_vcpu = false;
                kvm_rip_write(vcpu, ctxt->eip);
+               if (r == EMULATE_DONE)
+                       kvm_vcpu_check_singlestep(vcpu, &r);
+               kvm_set_rflags(vcpu, ctxt->eflags);
        } else
                vcpu->arch.emulate_regs_need_sync_to_vcpu = true;