[PATCH 3/3] KVM: x86: handle singlestep during emulation

Paolo Bonzini
2013-07-30T15:11:51Z

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>
---
arch/x86/kvm/x86.c | 39 ++++++++++++++++++++++++++++++++++++++-
1 file changed, 38 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 1368cf5..9805cfd 100644
--- 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;

--
1.8.1.4