On Fri, Jan 25, 2019 at 9:52 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote: > > On 21/01/19 13:48, Alexander Popov wrote: > > 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 <alex.popov@xxxxxxxxx> > > --- > > 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) > > > > Queued, with Cc to stable and the right "Fixes" trailer. > > Paolo This patch seems to imply that userspace is expected to set the guest's RFLAGS.TF when requesting KVM_GUEST_SINGLESTEP, but the documentation for KVM_GUEST_SINGLESTEP is not clear about this. Can someone clarify how KVM_GUEST_SINGLESTEP is supposed to work?