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

From: Alexey Kardashevskiy
Subject: Re: [Qemu-ppc] [PATCH] spapr: support CPU hotplug
Date: Tue, 27 Aug 2013 19:03:20 +1000
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130625 Thunderbird/17.0.7

On 08/27/2013 06:54 PM, Alexander Graf wrote:
> 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.

cpu_interrupt(CPU_INTERRUPT_HALT) works fine for TCG but does not for KVM
(the rtas call returns to the guest and it reports BUG).


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