Re: [PATCH v3 06/11] x86/hvm: allowing registering EOI callbacks for GSIs

[Jan Beulich <jbeulich@suse.com>]

On 07.04.2021 19:08, Roger Pau Monné wrote:
> On Wed, Apr 07, 2021 at 05:51:14PM +0200, Jan Beulich wrote:
>> On 31.03.2021 12:32, Roger Pau Monne wrote:
>>> +bool hvm_gsi_has_callbacks(const struct domain *d, unsigned int gsi)
>>> +{
>>> +    struct hvm_irq *hvm_irq = hvm_domain_irq(d);
>>> +    bool has_callbacks;
>>> +
>>> +    read_lock(&hvm_irq->gsi_callbacks_lock);
>>> +    has_callbacks = !list_empty(&hvm_irq->gsi_callbacks[gsi]);
>>> +    read_unlock(&hvm_irq->gsi_callbacks_lock);
>>> +
>>> +    return has_callbacks;
>>> +}
>>
>> What use is this function? Its result is stale by the time the
>> caller can look at it, as you've dropped the lock.
> 
> Right, that function is only used to decide whether the vIOAPIC needs
> to register an EOI callback when injecting a vector to the vlapic. The
> workflow is to first register a callback with the vIOAPIC and
> afterwards inject an interrupt which will trigger the callback
> logic.
> 
> Playing with the callback registration while interrupts can be
> injected will likely result in a malfunction of the device that relies
> on those callbacks, but that's to be expected anyway when playing such
> games.
> 
> That said multiple users sharing a vIOAPIC pin should be fine as long
> as they follow the logic above: always register a callback before
> attempting to inject an interrupt.

May I ask that you add a comment ahead of this function pointing out
the restriction?

>>> @@ -443,7 +457,8 @@ static void ioapic_inj_irq(
>>>      struct vlapic *target,
>>>      uint8_t vector,
>>>      uint8_t trig_mode,
>>> -    uint8_t delivery_mode)
>>> +    uint8_t delivery_mode,
>>> +    bool callback)
>>>  {
>>>      HVM_DBG_LOG(DBG_LEVEL_IOAPIC, "irq %d trig %d deliv %d",
>>>                  vector, trig_mode, delivery_mode);
>>> @@ -452,7 +467,7 @@ static void ioapic_inj_irq(
>>>             (delivery_mode == dest_LowestPrio));
>>>  
>>>      vlapic_set_irq_callback(target, vector, trig_mode,
>>> -                            trig_mode ? eoi_callback : NULL, NULL);
>>> +                            callback ? eoi_callback : NULL, NULL);
>>
>> I think you'd better use trig_mode || callback here and ...
>>
>>> @@ -466,6 +481,7 @@ static void vioapic_deliver(struct hvm_vioapic *vioapic, unsigned int pin)
>>>      struct vlapic *target;
>>>      struct vcpu *v;
>>>      unsigned int irq = vioapic->base_gsi + pin;
>>> +    bool callback = trig_mode || hvm_gsi_has_callbacks(d, irq);
>>>  
>>>      ASSERT(spin_is_locked(&d->arch.hvm.irq_lock));
>>>  
>>> @@ -492,7 +508,8 @@ static void vioapic_deliver(struct hvm_vioapic *vioapic, unsigned int pin)
>>>              target = vlapic_lowest_prio(d, NULL, 0, dest, dest_mode);
>>>          if ( target != NULL )
>>>          {
>>> -            ioapic_inj_irq(vioapic, target, vector, trig_mode, delivery_mode);
>>> +            ioapic_inj_irq(vioapic, target, vector, trig_mode, delivery_mode,
>>> +                           callback);
>>
>> ... invoke hvm_gsi_has_callbacks() right here and ...
>>
>>> @@ -507,7 +524,7 @@ static void vioapic_deliver(struct hvm_vioapic *vioapic, unsigned int pin)
>>>          for_each_vcpu ( d, v )
>>>              if ( vlapic_match_dest(vcpu_vlapic(v), NULL, 0, dest, dest_mode) )
>>>                  ioapic_inj_irq(vioapic, vcpu_vlapic(v), vector, trig_mode,
>>> -                               delivery_mode);
>>> +                               delivery_mode, callback);
>>
>> ... here, avoiding to call the function when you don't need the
>> result.
> 
> I think there's a slim chance of not needing to use the callback local
> variable, and hence didn't consider limiting it. I can do, but I'm
> unsure this will bring any real benefit while making the code more
> complex IMO.

Really the variable remaining unused in a minor set of cases was only
a secondary observation. What I first stumbled over is the moving of
the decision whether a callback is wanted from ioapic_inj_irq() to its
caller. Since the function clearly is intended as a helper of
vioapic_deliver(), I guess in the end it's fine the way you have it.

Jan