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

[David Hildenbrand]

On 24.07.23 10:42, Ilya Leoshkevich wrote:
> 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@xxxxxxxxxxxxx>
> > > ---
> > >    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?

Could be, there are many possible corner cases (PGM interrupt at the 
first instruction of PGM interrupt handler -- our PSW address might not 
even change)

--
Cheers,

David / dhildenb