Re: [Qemu-devel] [PATCH pic32 2/7] Stop simulation when processor is suspended forever by WAIT instruction with interrupts disabled.

[Peter Crosthwaite]

On Tue, Jun 30, 2015 at 6:57 PM, Serge Vakulenko <address@hidden> wrote:
> Hi Peter and Leon,
>
> With a bit of thinking, I agree, that the question of session
> termination on WAIT instruction is quite complicated in case of
> multi-core system, background i/o activity, mipsR6 core etc. So I'm
> going to find another solution for the task. What I essentially want
> here is to stop the simulator when the target Unix system is halted,
> like:
>
> $ /usr/local/qemu-mips/bin/qemu-system-mipsel -M pic32mx7-max32
> -nographic -monitor none -serial stdio -bios boot-max32.hex -kernel
> unix.hex -sd sdcard.img
> Board: chipKIT Max32
> Processor: M4K
> RAM size: 128 kbytes
> Load file: 'boot-max32.hex', 6720 bytes
> Load file: 'unix.hex', 144992 bytes
> Card0 image 'sdcard.img', 102888 kbytes
> [...]
> 2.11 BSD UNIX (pic32) (console)
>
> login: root
> Password:
> Welcome to RetroBSD!
> erase, kill ^U, intr ^C
> # halt
> killing processes... done
> syncing disks... done
> halted
> $ _                           <-- QEMU terminated
>
> On BSD, the halt command uses reboot(RB_HALT) system call to terminate
> the operating system. It essentially results in an endless loop on
> wait instruction with interrupts disabled., like "for(;;) {
> asm("wait"); }". For pic32 it makes little sense to continue
> simulation in this case.
>

What about releasing the hung state from a debugger? I think this
auto-exit behavior should be optional and not a default.

> Fortunately, I've found a solution which does not require modification
> of generic code. Everything can be done in the platform-specific part.
>

But I think your desire is for a generic feature - stopping emulation
on a hung state.

Regards,
Peter

> Thanks,
> --Serge
>
>
> 2015-06-30 11:08 GMT-07:00 Peter Crosthwaite <address@hidden>:
>> On Tue, Jun 30, 2015 at 7:10 AM, Leon Alrae <address@hidden> wrote:
>>> Hi Serge,
>>>
>>> On 30/06/2015 06:02, Serge Vakulenko wrote:
>>>> Signed-off-by: Serge Vakulenko <address@hidden>
>>>> ---
>>>>  target-mips/op_helper.c | 7 +++++++
>>>>  1 file changed, 7 insertions(+)
>>>>
>>>> diff --git a/target-mips/op_helper.c b/target-mips/op_helper.c
>>>> index 2a9ddff..1b7caeb 100644
>>>> --- a/target-mips/op_helper.c
>>>> +++ b/target-mips/op_helper.c
>>>> @@ -22,6 +22,7 @@
>>>>  #include "exec/helper-proto.h"
>>>>  #include "exec/cpu_ldst.h"
>>>>  #include "sysemu/kvm.h"
>>>> +#include "sysemu/sysemu.h"
>>>>
>>>>  #ifndef CONFIG_USER_ONLY
>>>>  static inline void cpu_mips_tlb_flush (CPUMIPSState *env, int 
>>>> flush_global);
>>>> @@ -2235,6 +2236,12 @@ void helper_wait(CPUMIPSState *env)
>>>>  {
>>>>      CPUState *cs = CPU(mips_env_get_cpu(env));
>>>>
>>>> +#ifndef CONFIG_USER_ONLY
>>>> +    if (!(env->CP0_Status & (1 << CP0St_IE))) {
>>>> +        /* WAIT instruction with interrupts disabled - halt the 
>>>> simulation. */
>>>> +        qemu_system_shutdown_request();
>>>> +    }
>>>> +#endif
>>>
>>> Why do you want to stop the simulation, wouldn't it be beneficial to leave 
>>> it
>>> running? For debugging for example, the user would be still able to inspect
>>> the state, that could help to find a clue why CPU got suspended forever.
>>>
>>
>> In theory, there may also be other actors in the system. Even if the
>> CPU is in a never-going-to-come-back-to-life state, other-things may
>> still be happening. This would be best as some sort of core feature to
>> detect total-inaction of the system. That is, there are no delayed
>> events pending (timed events and AIOs) and the CPUs are halted. Then
>> this would be a user option to stop the system on reaching the
>> machine-inactive state and it would be applicable beyond mips.
>>
>> Regards,
>> Peter
>>
>>> Also, if we take into account implementations (currently not supported in
>>> QEMU) where CPU can be woken up by a disabled interrupt (the Config7.WII bit
>>> in P5600 for example), then this won't be correct.
>>>
>>> Thanks,
>>> Leon
>>>
>>>
>