Re: [Qemu-devel] MTTCG External Halt

[Alistair Francis]

On Thu, Feb 1, 2018 at 9:13 AM, Alistair Francis
<address@hidden> wrote:
> On Thu, Feb 1, 2018 at 4:01 AM, Alex Bennée <address@hidden> wrote:
>>
>> Alistair Francis <address@hidden> writes:
>>
>>> On Wed, Jan 31, 2018 at 12:32 PM, Alex Bennée <address@hidden> wrote:
>>>>
>>>> Alistair Francis <address@hidden> writes:
>>>>
>>>>> On Tue, Jan 30, 2018 at 8:26 PM, Paolo Bonzini <address@hidden> wrote:
>>>>>> On 30/01/2018 18:56, Alistair Francis wrote:
>>>>>>>
>>>>>>> I don't have a good solution though, as setting CPU_INTERRUPT_RESET
>>>>>>> doesn't help (that isn't handled while we are halted) and
>>>>>>> async_run_on_cpu()/run_on_cpu() doesn't reliably reset the CPU when we
>>>>>>> want.
>>>>>>>
>>>>>>> I've ever tried pausing all CPUs before reseting the CPU and them
>>>>>>> resuming them all but that doesn't seem to to work either.
>>>>>>
>>>>>> async_safe_run_on_cpu would be like async_run_on_cpu, except that it
>>>>>> takes care of stopping all other CPUs while the function runs.
>>>>>>
>>>>>>> Is there
>>>>>>> anything I'm missing? Is there no reliable way to reset a CPU?
>>>>>>
>>>>>> What do you mean by reliable?  Executing no instruction after the one
>>>>>> you were at?
>>>>>
>>>>> The reset is called by a GPIO line, so I need the reset to be called
>>>>> basically as quickly as the GPIO line changes. The async_ and
>>>>> async_safe_ functions seem to not run quickly enough, even if I run a
>>>>> process_work_queue() function afterwards.
>>>>>
>>>>> Is there a way to kick the CPU to act on the async_*?
>>>>
>>>> Define quickly enough? The async_(safe) functions kick the vCPUs so they
>>>> will all exit the run loop as they enter the next TB (even if they loop
>>>> to themselves).
>>>
>>> We have a special power controller CPU that wakes all the CPUs up and
>>> at boot the async_* functions don't wake the CPUs up. If I just use
>>> the cpu_rest() function directly everything starts fine (but then I
>>> hit issues later).
>>>
>>> If I forcefully run process_queued_cpu_work() then I can get the CPUs
>>> up, but I don't think that is the right solution.
>>>
>>>>
>>>> From an external vCPUs point of view those extra instructions have
>>>> already executed. If the resetting vCPU needs them to have reset by the
>>>> time it executes it's next instruction it should either cpu_loop_exit at
>>>> that point or ensure it is the last instruction in it's TB (which is
>>>> what we do for the MMU flush cases in ARM, they all end the TB at that
>>>> point).
>>>
>>> cpu_loop_exit() sounds like it would help, but as I'm not in the CPU
>>> context it just seg faults.
>>
>> What context are you in? gdb-stub does have to something like this.
>
> gdb-stub just seems to use vm_stop() and vm_start().
>
> That fixes all hangs/asserts, but now Linux only brings up 1 CPU (instead of 
> 4).

Hmmm... Interesting if I do this on reset events:

        pause_all_vcpus();
        cpu_reset(cpu);
        resume_all_vcpus();

it hangs, while if I do this

        if (runstate_is_running()) {
            vm_stop(RUN_STATE_PAUSED);
        }
        cpu_reset(cpu);
        if (!runstate_needs_reset()) {
            vm_start();
        }

it doesn't hang but CPU bringup doesn't work.

Alistair

>
> Alistair