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Date: Mon, 21 Jan 2019 15:48:40 +0300
From: Alexander Popov <>
To: Paolo Bonzini <>,
	Radim Krcmar <>,
	Thomas Gleixner <>,
	Ingo Molnar <>,
	Borislav Petkov <>,
	Nadav Amit <>,
	Andy Lutomirski <>,
	Stefan Hajnoczi <>,
	H Peter Anvin <>,,,,,
	Alexander Popov <>
Subject: [PATCH v2] KVM: x86: Fix single-step debugging

The single-step debugging of KVM guests on x86 is broken: if we run
gdb 'stepi' command at the breakpoint when the guest interrupts are
enabled, RIP always jumps to native_apic_mem_write(). Then other
nasty effects follow.

Long investigation showed that on Jun 7, 2017 the
commit c8401dda2f0a00cd25c0 ("KVM: x86: fix singlestepping over syscall")
introduced the kvm_run.debug corruption: kvm_vcpu_do_singlestep() can
be called without X86_EFLAGS_TF set.

Let's fix it. Please consider that for -stable.

Signed-off-by: Alexander Popov <>
 arch/x86/kvm/x86.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index f049ecf..9686068 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6407,8 +6407,7 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
 		toggle_interruptibility(vcpu, ctxt->interruptibility);
 		vcpu->arch.emulate_regs_need_sync_to_vcpu = false;
 		kvm_rip_write(vcpu, ctxt->eip);
-		if (r == EMULATE_DONE &&
-		    (ctxt->tf || (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)))
+		if (r == EMULATE_DONE && ctxt->tf)
 			kvm_vcpu_do_singlestep(vcpu, &r);
 		if (!ctxt->have_exception ||
 		    exception_type(ctxt->exception.vector) == EXCPT_TRAP)

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