According to APM, the #DB intercept for a single-stepped VMRUN must happen
after the completion of that instruction, when the guest does #VMEXIT to
the host. However, in the current implementation of KVM, the #DB intercept
for a single-stepped VMRUN happens after the completion of the instruction
that follows the VMRUN instruction. When the #DB intercept handler is
invoked, it shows the RIP of the instruction that follows VMRUN, instead of
of VMRUN itself. This is an incorrect RIP as far as single-stepping VMRUN
is concerned.

This patch fixes the problem by checking, in nested_svm_vmexit(), for the
condition that the VMRUN instruction is being single-stepped and if so,
queues the pending #DB intercept so that the #DB is accounted for before
we execute L1's next instruction.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Krish Sadhukhan <redacted>
---
  arch/x86/kvm/svm/nested.c | 10 ++++++++++
  1 file changed, 10 insertions(+)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 35891d9a1099..713ce5cfb0db 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c

@@ -720,6 +720,16 @@ int nested_svm_vmexit(struct vcpu_svm *svm)
  	kvm_clear_exception_queue(&svm->vcpu);
  	kvm_clear_interrupt_queue(&svm->vcpu);
  
+	/*
+	 * If we are here following the completion of a VMRUN that
+	 * is being single-stepped, queue the pending #DB intercept
+	 * right now so that it an be accounted for before we execute
+	 * L1's next instruction.
+	 */
+	if (unlikely(svm->vmcb->control.exit_code == SVM_EXIT_VMRUN &&
+	    svm->vmcb->save.rflags & X86_EFLAGS_TF))
+		kvm_queue_exception(&(svm->vcpu), DB_VECTOR);
+
  	return 0;
  }