Re: [Qemu-ppc] [PATCH] spapr: support CPU hotplug

[Alexander Graf]

On 27.08.2013, at 09:41, Alexey Kardashevskiy wrote:

> On 08/27/2013 05:02 PM, Paolo Bonzini wrote:
>> Il 27/08/2013 08:37, Alexey Kardashevskiy ha scritto:
>>>>> So this is here to make sure we don't accidentally get out of halted 
>>>>> state by an interrupt on that vcpu. Could you please somehow make that 
>>>>> part obvious? Either by adding a comment or by only explicitly masking 
>>>>> DEC and EE and a comment :).
>>>>> 
>>>>>> +    cs->exit_request = 1;
>>>>> 
>>>>> This should probably be qemu_cpu_kick_self().
>>>> 
>>>> Uh, no, I don't think so.  This is there purely to make sure we exit
>>>> the inner loop, and actually test cpu_can_run() which will test
>>>> halted.  AFAICT qemu_cpu_kick_self() won't do anything similar.
>>> 
>>> rtas_stop_self() eventually returns to kvm_cpu_exec() which calls
>>> qemu_cpu_kick_self() and resets cs->exit_request before return so I do not
>>> really see the difference in behaviour. And actually both ways CPU stops in
>>> exactly the same way. What do I miss?
>> 
>> What about TCG?
> 
> Oh. Right. TCG :(
> 
> qemu_cpu_kick_self() crashes the guest and cs->exit_request works fine.
> 
> Why? Both should work? What is the expected behavior here? Thanks.

Hrm. To me exit_request always was an internal piece of state that the inner 
loop uses to find out whether to exit, but not something we should randomly set 
from a device (and hypercalls / rtas calls are very similar to devices). So I 
would like to not have any code in hw/ that modifies it.

However, we need the functionality of breaking out of the main loop, I agree.

Maybe what you are really looking for is cpu_interrupt(CPU_INTERRUPT_HALT). 
That sets halted = 1 and exits the main loop, because it's an interrupt.


Alex