Re: [PATCH V1] gdbstub: suspended state support

[Alex Bennée]

Steven Sistare <steven.sistare@oracle.com> writes:

> On 1/7/2021 7:40 AM, Alex Bennée wrote:
>> Steve Sistare <steven.sistare@oracle.com> writes:
>> 
>>> Modify the gdb server so a continue command appears to resume execution
>>> when in RUN_STATE_SUSPENDED.  Do not print the next gdb prompt, but do not
>>> actually resume instruction fetch.  While in this "fake" running mode, a
>>> ctrl-C returns the user to the gdb prompt.
>> 
>> What exactly is the purpose of this? To hide the details of the runstate
>> as controlled by the user? I wouldn't expect someone using gdb debugging
>> not to also have control of the HMP/QMP interface.
>
> Without this fix, a user that attaches gdb to a suspended guest breaks the
> guest.  The state is RUN_STATE_SUSPENDED.  After attaching gdb and typing
> continue or quit, qemu transitions to RUN_STATE_RUNNING (wrong) and the
> guest continues execution (wrong).  The guest loops polling on an acpi port,
> deep in a call stack under acpi_suspend_enter().  Sending a system_wakeup
> request via qmp or hmp fails with the message "Error: Unable to wake up:
> guest is not in suspended state".
>
> With the fix, the state remains RUN_STATE_SUSPENDED throughout, until the
> system_wakeup request, and the guest pc does not change.  gdb interprets 
> RUN_STATE_SUSPENDED as "target is running", without causing instruction 
> fetch to resume.
>
> If you are satisfied, I will add this explanation to the commit
> message.

I'm satisfied with the explanation going in the commit message. However
I'm not convinced the implementation of pretending we worked. I think if
we are not going to start we should probably reply with an O packet
explaining why we don't start followed by a S (stop) packet so GDB
doesn't get confused.

I would also be happier if we could add a test case that works through
all the potential state transitions so we don't have test manually (i.e.
not). I think we need at least:

   - -S -> stop -> continue -> start -> Ctrl-C
   - -S -> continue -> stop -> start -> Ctrl-C
   - Ctrl-C -> stop -> continue -> start
   - stop -> Ctrl-C -> start -> continue 

I suspect it would need to use the acceptance tests given you'll want to
change two control points. The reverse debugging tests already do
something similar (see tests/acceptance/reverse_debugging.py).

>
> - Steve
>
>>> Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
>>> ---
>>>  gdbstub.c | 11 +++++++++--
>>>  1 file changed, 9 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/gdbstub.c b/gdbstub.c
>>> index f3a318c..2f0d9ff 100644
>>> --- a/gdbstub.c
>>> +++ b/gdbstub.c
>>> @@ -461,7 +461,9 @@ static inline void gdb_continue(void)
>>>  #else
>>>      if (!runstate_needs_reset()) {
>>>          trace_gdbstub_op_continue();
>>> -        vm_start();
>>> +        if (!runstate_check(RUN_STATE_SUSPENDED)) {
>>> +            vm_start();
>>> +        }
>>>      }
>>>  #endif
>>>  }
>>> @@ -490,7 +492,7 @@ static int gdb_continue_partial(char *newstates)
>>>      int flag = 0;
>>>  
>>>      if (!runstate_needs_reset()) {
>>> -        if (vm_prepare_start()) {
>>> +        if (!runstate_check(RUN_STATE_SUSPENDED) && vm_prepare_start()) {
>>>              return 0;
>>>          }
>>>  
>>> @@ -2835,6 +2837,9 @@ static void gdb_read_byte(uint8_t ch)
>>>          /* when the CPU is running, we cannot do anything except stop
>>>             it when receiving a char */
>>>          vm_stop(RUN_STATE_PAUSED);
>>> +    } else if (runstate_check(RUN_STATE_SUSPENDED) && ch == 3) {
>>> +        /* Received ctrl-c from gdb */
>>> +        gdb_vm_state_change(0, 0, RUN_STATE_PAUSED);
>>>      } else
>>>  #endif
>>>      {
>>> @@ -3282,6 +3287,8 @@ static void gdb_sigterm_handler(int signal)
>>>  {
>>>      if (runstate_is_running()) {
>>>          vm_stop(RUN_STATE_PAUSED);
>>> +    } else if (runstate_check(RUN_STATE_SUSPENDED)) {
>>> +        gdb_vm_state_change(0, 0, RUN_STATE_PAUSED);
>>>      }
>>>  }
>>>  #endif
>> 
>> 


-- 
Alex Bennée