Re: [PATCH v2] KVM: x86: Fix single-step debugging

[Jim Mattson]

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?