Re: [Qemu-devel] MTTCG External Halt

[Alex Bennée]

Alistair Francis <address@hidden> writes:

> 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.

Hmm I'm still confused what context you are in. Is this an externally
triggered reset via the (qemu) prompt or something?

>
> Alistair
>
>>
>> Alistair


--
Alex Bennée