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

[David Hildenbrand <david@redhat.com>]

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@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?

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