Re: [PATCH 2/2] KVM : powerpc/booke: Allow debug interrupt injection to guest

[Alexander Graf <agraf@suse.de>]

On 01.07.14 12:06, Bharat.Bhushan@freescale.com wrote:
>
>> -----Original Message-----
>> From: Alexander Graf [mailto:agraf@suse.de]
>> Sent: Tuesday, July 01, 2014 11:53 AM
>> To: Wood Scott-B07421
>> Cc: Bhushan Bharat-R65777; kvm-ppc@vger.kernel.org; kvm@vger.kernel.org
>> Subject: Re: [PATCH 2/2] KVM : powerpc/booke: Allow debug interrupt injection to
>> guest
>>
>>
>>
>>> Am 30.06.2014 um 22:25 schrieb Scott Wood <scottwood@freescale.com>:
>>>
>>>> On Sun, 2014-06-29 at 23:38 -0500, Bhushan Bharat-R65777 wrote:
>>>>
>>>>> -----Original Message-----
>>>>> From: Wood Scott-B07421
>>>>> Sent: Friday, June 27, 2014 11:53 PM
>>>>> To: Bhushan Bharat-R65777
>>>>> Cc: agraf@suse.de; kvm-ppc@vger.kernel.org; kvm@vger.kernel.org
>>>>> Subject: Re: [PATCH 2/2] KVM : powerpc/booke: Allow debug interrupt
>>>>> injection to guest
>>>>>
>>>>>> On Fri, 2014-06-27 at 11:55 +0530, Bharat Bhushan wrote:
>>>>>> -    /* Force enable debug interrupts when user space wants to debug */
>>>>>> -    if (vcpu->guest_debug) {
>>>>>> +    /*
>>>>>> +     * Force enable debug interrupts when user space wants to debug
>>>>>> +     * and there is no debug interrupt pending for guest to handle.
>>>>>> +     */
>>>>>> +    if (vcpu->guest_debug && !kvmppc_core_pending_debug(vcpu)) {
>>>>> Are you trying to allow the guest to be simultaneously debugged by
>>>>> itself and by host userspace?  How does this work?
>>>> Not actually, Currently we are not partitioning debug resources
>>>> between host userspace and guest. In fact we do not emulate debug
>>>> registers for guest. But we want host userspace to pass the interrupt
>>>> to guest if it is not able to handle.
>>> I don't understand the logic here.  A debug interrupt should be
>>> injected when the programming model in the guest says that a debug
>>> interrupt should happen.  How can that occur currently?  If the guest
>>> didn't set up the debug registers and QEMU still can't handle the
>>> debug interrupt, that's a bug in QEMU (or KVM, or the hardware...).
>>> Injecting the interrupt into the guest just adds another bug on top of that.
>> I don't think QEMU should be aware of these limitations.
>>
>>>>>> #ifdef CONFIG_KVM_BOOKE_HV
>>>>>>         /*
>>>>>>          * Since there is no shadow MSR, sync MSR_DE into the guest
>>>>>> @@
>>>>>> -264,6 +272,16 @@ static void kvmppc_core_dequeue_watchdog(struct
>>>>>> kvm_vcpu
>>>>> *vcpu)
>>>>>>     clear_bit(BOOKE_IRQPRIO_WATCHDOG,
>>>>>> &vcpu->arch.pending_exceptions); }
>>>>>>
>>>>>> +static void kvmppc_core_queue_debug(struct kvm_vcpu *vcpu) {
>>>>>> +    kvmppc_booke_queue_irqprio(vcpu, BOOKE_IRQPRIO_DEBUG); }
>>>>>> +
>>>>>> +static void kvmppc_core_dequeue_debug(struct kvm_vcpu *vcpu) {
>>>>>> +    clear_bit(BOOKE_IRQPRIO_DEBUG,
>>>>>> +&vcpu->arch.pending_exceptions); }
>>>>> Is there currently no support for a guest debugging itself (i.e.
>>>>> guest_debug unset) on e500v2?
>>>> Yes, It is not yet supported (IACx/DACx/DBCR/DBSR/DSRRx are not yet
>> emulated).
>>> How is it useful to inject a debug exception into the guest, until
>>> these things are emulated?
>> We don't have to touch QEMU later then ;). But I agree that it would make a lot
>> of sense to enable guest debugging along the way - it can't be that hard, no?
> Copy pasting my response in another email:
>
> "
> Ok, Till we add support for guest to used debug resource, can we say that userspace will still try to inject debug interrupt (as it does not know guest capability) to guest but KVM will:
>   - clear guest dbsr

Can we fake the value of DBSR that the guest sees?


Alex