Re: [PATCH v3 04/10] VFIO: platform: add vfio_platform_set_automasked

[Antonios Motakis <antonios.motakis@huawei.com>]

On 18-Aug-15 19:44, Alex Williamson wrote:
> On Mon, 2015-08-17 at 17:38 +0200, Eric Auger wrote:
>> On 08/12/2015 08:56 PM, Alex Williamson wrote:
>>> On Mon, 2015-08-10 at 15:20 +0200, Eric Auger wrote:
>>>> This function makes possible to change the automasked mode.
>>>>
>>>> Signed-off-by: Eric Auger <eric.auger@linaro.org>
>>>>
>>>> ---
>>>>
>>>> v1 -> v2:
>>>> - set forwarded flag
>>>> ---
>>>>  drivers/vfio/platform/vfio_platform_irq.c | 19 +++++++++++++++++++
>>>>  1 file changed, 19 insertions(+)
>>>>
>>>> diff --git a/drivers/vfio/platform/vfio_platform_irq.c b/drivers/vfio/platform/vfio_platform_irq.c
>>>> index b31b1f0..a285384 100644
>>>> --- a/drivers/vfio/platform/vfio_platform_irq.c
>>>> +++ b/drivers/vfio/platform/vfio_platform_irq.c
>>>> @@ -186,6 +186,25 @@ static irqreturn_t vfio_handler(int irq, void *dev_id)
>>>>  	return ret;
>>>>  }
>>>>  
>>>> +static int vfio_platform_set_automasked(struct vfio_platform_irq *irq,
>>>> +					   bool automasked)
>>>> +{
>>>> +	unsigned long flags;
>>>> +
>>>> +	spin_lock_irqsave(&irq->lock, flags);
>>>> +	if (automasked) {
>>>> +		irq->forwarded = true;
>>>> +		irq->flags |= VFIO_IRQ_INFO_AUTOMASKED;
>>>> +		irq->handler = vfio_automasked_irq_handler;
>>>> +	} else {
>>>> +		irq->forwarded = false;
>>>> +		irq->flags &= ~VFIO_IRQ_INFO_AUTOMASKED;
>>>> +		irq->handler = vfio_irq_handler;
>>>> +	}
>>>> +	spin_unlock_irqrestore(&irq->lock, flags);
>>>> +	return 0;
>>>
>>> In vfio-speak, automasked means level and we're not magically changing
>>> the IRQ from level to edge, we're simply able to handle level
>>> differently based on a hardware optimization.  Should the user visible
>>> flags therefore change based on this?  Aren't we really setting the
>>> forwarded state rather than the automasked state?
>>
>> Well actually this was following the discussion we had a long time ago
>> about that topic:
>>
>> http://lkml.iu.edu/hypermail/linux/kernel/1409.1/03659.html
>>
>> I did not really know how to conclude ...
>>
>> If it is preferred I can hide this to the userspace, no problem.
> 
> I think that was based on the user being involved in enabling forwarding
> though, now that it's hidden and automatic, it doesn't make much sense
> to me to toggle any of the interrupt info details based on the state of
> the forward.  The user always needs to handle the interrupt as level
> since the bypass can be torn down at any point in time.  We're taking
> advantage of the in-kernel path to make further optimizations, which
> seems like they should be transparent to the user.  Thanks,

I wonder if it makes sense to rename VFIO_IRQ_INFO_AUTOMASKED to
VFIO_IRQ_INFO_LEVEL_TRIGGERED, and reintroduce VFIO_IRQ_INFO_AUTOMASKED as
an alias, so compatibility with user space can be maintained? This way
this semantic misunderstanding could be left behind.

Cheers,
Antonios

> 
> Alex
> 
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-- 
Antonios Motakis
Virtualization Engineer
Huawei Technologies Duesseldorf GmbH
European Research Center
Riesstrasse 25, 80992 München