Re: [PATCH 4/4 v2] KVM: PPC: Add userspace debug stub support

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Am 03.04.2013 um 12:03 schrieb Bhushan Bharat-R65777 <R65777@xxxxxxxxxxxxx>:

> 
> 
>> -----Original Message-----
>> From: Wood Scott-B07421
>> Sent: Tuesday, April 02, 2013 11:30 PM
>> To: Bhushan Bharat-R65777
>> Cc: Alexander Graf; kvm-ppc@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; Wood Scott-
>> B07421
>> Subject: Re: [PATCH 4/4 v2] KVM: PPC: Add userspace debug stub support
>> 
>> On 04/02/2013 09:09:34 AM, Bhushan Bharat-R65777 wrote:
>>> 
>>> 
>>>> -----Original Message-----
>>>> From: Alexander Graf [mailto:agraf@xxxxxxx]
>>>> Sent: Tuesday, April 02, 2013 1:57 PM
>>>> To: Bhushan Bharat-R65777
>>>> Cc: kvm-ppc@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; Wood Scott-B07421
>>>> Subject: Re: [PATCH 4/4 v2] KVM: PPC: Add userspace debug stub
>>> support
>>>> 
>>>> 
>>>> On 29.03.2013, at 07:04, Bhushan Bharat-R65777 wrote:
>>>> 
>>>>> 
>>>>> 
>>>>>> -----Original Message-----
>>>>>> From: Alexander Graf [mailto:agraf@xxxxxxx]
>>>>>> Sent: Thursday, March 28, 2013 10:06 PM
>>>>>> To: Bhushan Bharat-R65777
>>>>>> Cc: kvm-ppc@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; Wood
>>> Scott-B07421;
>>>>>> Bhushan
>>>>>> Bharat-R65777
>>>>>> Subject: Re: [PATCH 4/4 v2] KVM: PPC: Add userspace debug stub
>>>>>> support
>>>>>> 
>>>>>> 
>>>>>> How does the normal debug register switching code work in Linux?
>>>>>> Can't we just reuse that? Or rely on it to restore working state
>>> when
>>>>>> another process gets scheduled in?
>>>>> 
>>>>> Good point, I can see debug registers loading in function
>>> __switch_to()-
>>>>> switch_booke_debug_regs() in file arch/powerpc/kernel/process.c.
>>>>> So as long as assume that host will not use debug resources we
>>> can rely on
>>>> this restore. But I am not sure that this is a fare assumption. As
>>> Scott earlier
>>>> mentioned someone can use debug resource for kernel debugging also.
>>>> 
>>>> Someone in the kernel can also use floating point registers. But
>>> then it's his
>>>> responsibility to clean up the mess he leaves behind.
>>> 
>>> I am neither convinced by what you said and nor even have much reason
>>> to oppose :)
>>> 
>>> Scott,
>>>    I remember you mentioned that host can use debug resources, you
>>> comment on this ?
>> 
>> I thought the conclusion we reached was that it was OK as long as KVM waits
>> until it actually needs the debug resources to mess with the registers.
> 
> Right,  Are we also agreeing on that KVM will not save/restore host debug context on vcpu_load/vcpu_put()? KVM will load its context in vcpu_load() if needed and on vcpu_put() it will clear DBCR0 and DBSR.

That depends on whether the kernel restores the debug registers. Please double-check that.

Also, someone could want to gdb QEMU, so the debug registers might have to get restored on a heavy weight exit. I'd hope Linux just provides helpers to restore a process's debug state that we can call here.


Alex

> 
> Thanks
> -Bharat
> 
> 
> 
> 
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