Re: [PATCH 3/4] Watchdog exit handling support

[Scott Wood <scottwood@freescale.com>]

On 07/05/2012 07:43 PM, Bhushan Bharat-R65777 wrote:
> 
> 
>> -----Original Message-----
>> From: Wood Scott-B07421
>> Sent: Friday, July 06, 2012 1:57 AM
>> To: Bhushan Bharat-R65777
>> Cc: Wood Scott-B07421; qemu-ppc@nongnu.org; kvm-ppc@vger.kernel.org;
>> agraf@suse.de
>> Subject: Re: [PATCH 3/4] Watchdog exit handling support
>>
>> On 07/04/2012 06:13 AM, Bhushan Bharat-R65777 wrote:
>>>
>>>
>>>> -----Original Message-----
>>>> From: Wood Scott-B07421
>>>> Sent: Friday, June 29, 2012 3:57 AM
>>>> To: Bhushan Bharat-R65777
>>>> Cc: qemu-ppc@nongnu.org; kvm-ppc@vger.kernel.org; agraf@suse.de;
>>>> Bhushan Bharat-
>>>> R65777
>>>> Subject: Re: [PATCH 3/4] Watchdog exit handling support
>>>>
>>>> On 06/28/2012 12:39 AM, Bharat Bhushan wrote:
>>>>> This patch adds the support to handle the exit caused by watchdog
>>>>> (KVM_EXIT_WDT). In the handling we clear the TSR register.
>>>>
>>>> I'm not sure what the logical split is between this patch and 4/4...
>>>
>>> Ok I will merge 3/4 and 4/4.
>>>
>>>>
>>>>> diff --git a/hw/ppc_booke.c b/hw/ppc_booke.c index 837a5b6..a9fba15
>>>>> 100644
>>>>> --- a/hw/ppc_booke.c
>>>>> +++ b/hw/ppc_booke.c
>>>>> @@ -203,6 +203,11 @@ static void booke_wdt_cb(void *opaque)
>>>>>                               booke_timer->wdt_timer);  }
>>>>>
>>>>> +void ppc_booke_wdt_clear_tsr(CPUPPCState *env, target_ulong tsr) {
>>>>> +    env->spr[SPR_BOOKE_TSR] = tsr & ~(TSR_ENW | TSR_WIS |
>>>>> +TSR_WRS_MASK); }
>>>>> +
>>>>
>>>> We should probably call this function before returning to KVM, at
>>>> least after we halt for debug, possibly other times.
>>>
>>> Why clearing watchdog related bits in TSR? This will just reset the
>>> watchdog state machine. Do we actually want to reset the state machine
>>> or want watchdog interrupt to not occur when exiting to KVM for debug.
>>
>> We want to prevent long delays in QEMU (especially for debug halt) from causing
>> watchdog actions (interrupt, reset, etc).  Resetting the state machine seems
>> like the way to do that.
>>
>>> Does possibly other times mean except KVM_RUN?
>>
>> Not sure what you mean here.  Don't we always use KVM_RUN to enter the guest?
>> My point was we definitely want this after we had the guest halted, and we may
>> want to consider doing it for some other heavyweight exits.
> 
> Your initial comment was "We should probably call this function
> before returning to KVM". Which suggest that ioctls calls made by
> QEMU to enter KVM. I am a bit confused, now you want to avoid long
> delays in QEMU? This suggest whenever we exit to qemu, right?

Whenever we exit to QEMU is one possibility, though it's probably too
aggressive (if the guest gets stuck in a loop that contains a QEMU exit,
the watchdog won't catch it).  Another option is to reset the state
machine only when the guest resumes from a debug halt.

-Scott