Re: [PATCH v2 1/6] KVM: s390: interrupt: Fix single-stepping into interrupt handlers

[Ilya Leoshkevich <iii@linux.ibm.com>]

On Mon, 2023-07-24 at 10:22 +0200, David Hildenbrand wrote:
> On 21.07.23 13:57, Ilya Leoshkevich wrote:
> > After single-stepping an instruction that generates an interrupt,
> > GDB
> > ends up on the second instruction of the respective interrupt
> > handler.
> > 
> > The reason is that vcpu_pre_run() manually delivers the interrupt,
> > and
> > then __vcpu_run() runs the first handler instruction using the
> > CPUSTAT_P flag. This causes a KVM_SINGLESTEP exit on the second
> > handler
> > instruction.
> > 
> > Fix by delaying the KVM_SINGLESTEP exit until after the manual
> > interrupt delivery.
> > 
> > Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> > ---
> >   arch/s390/kvm/interrupt.c | 10 ++++++++++
> >   arch/s390/kvm/kvm-s390.c  |  4 ++--
> >   2 files changed, 12 insertions(+), 2 deletions(-)

[...]
> 

> Can we add a comment like
> 
> /*
>   * We delivered at least one interrupt and modified the PC. Force a
>   * singlestep event now.
>   */

Ok, will do.

> > +       if (delivered && guestdbg_sstep_enabled(vcpu)) {
> > +               struct kvm_debug_exit_arch *debug_exit = &vcpu-
> > >run->debug.arch;
> > +
> > +               debug_exit->addr = vcpu->arch.sie_block->gpsw.addr;
> > +               debug_exit->type = KVM_SINGLESTEP;
> > +               vcpu->guest_debug |= KVM_GUESTDBG_EXIT_PENDING;
> >         }
> 
> I do wonder if we, instead, want to do this whenever we modify the
> PSW.
> 
> That way we could catch any PC changes and only have to add checks
> for 
> guestdbg_exit_pending().

Wouldn't this break a corner case where the first instruction of the
interrupt handler causes the same interrupt?

> But this is simpler and should work as well.
> 
> Acked-by: David Hildenbrand <david@redhat.com>