Re: [PATCH 05/10] VFIO: pci: Introduce direct EOI INTx interrupt handler

[Auger Eric <eric.auger@redhat.com>]

Hi Alex, Marc,

On 31/05/2017 20:24, Alex Williamson wrote:
> On Wed, 24 May 2017 22:13:18 +0200
> Eric Auger <eric.auger@redhat.com> wrote:
> 
>> We add two new fields in vfio_pci_irq_ctx struct: deoi and handler.
>> If deoi is set, this means the physical IRQ attached to the virtual
>> IRQ is directly deactivated by the guest and the VFIO driver does
>> not need to disable the physical IRQ and mask it at VFIO level.
>>
>> The handler pointer is set accordingly and a wrapper handler is
>> introduced that calls the chosen handler function.
>>
>> Signed-off-by: Eric Auger <eric.auger@redhat.com>
>>
>> ---
>> ---
>>  drivers/vfio/pci/vfio_pci_intrs.c   | 32 ++++++++++++++++++++++++++------
>>  drivers/vfio/pci/vfio_pci_private.h |  2 ++
>>  2 files changed, 28 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/vfio/pci/vfio_pci_intrs.c b/drivers/vfio/pci/vfio_pci_intrs.c
>> index d4d377b..06aa713 100644
>> --- a/drivers/vfio/pci/vfio_pci_intrs.c
>> +++ b/drivers/vfio/pci/vfio_pci_intrs.c
>> @@ -121,11 +121,8 @@ void vfio_pci_intx_unmask(struct vfio_pci_device *vdev)
>>  static irqreturn_t vfio_intx_handler(int irq, void *dev_id)
>>  {
>>  	struct vfio_pci_device *vdev = dev_id;
>> -	unsigned long flags;
>>  	int ret = IRQ_NONE;
>>  
>> -	spin_lock_irqsave(&vdev->irqlock, flags);
>> -
>>  	if (!vdev->pci_2_3) {
>>  		disable_irq_nosync(vdev->pdev->irq);
>>  		vdev->ctx[0].automasked = true;
>> @@ -137,14 +134,33 @@ static irqreturn_t vfio_intx_handler(int irq, void *dev_id)
>>  		ret = IRQ_HANDLED;
>>  	}
>>  
>> -	spin_unlock_irqrestore(&vdev->irqlock, flags);
>> -
>>  	if (ret == IRQ_HANDLED)
>>  		vfio_send_intx_eventfd(vdev, NULL);
>>  
>>  	return ret;
>>  }
>>  
>> +static irqreturn_t vfio_intx_handler_deoi(int irq, void *dev_id)
>> +{
>> +	struct vfio_pci_device *vdev = dev_id;
>> +
>> +	vfio_send_intx_eventfd(vdev, NULL);
>> +	return IRQ_HANDLED;
>> +}
>> +
>> +static irqreturn_t vfio_intx_wrapper_handler(int irq, void *dev_id)
>> +{
>> +	struct vfio_pci_device *vdev = dev_id;
>> +	unsigned long flags;
>> +	irqreturn_t ret;
>> +
>> +	spin_lock_irqsave(&vdev->irqlock, flags);
>> +	ret = vdev->ctx[0].handler(irq, dev_id);
>> +	spin_unlock_irqrestore(&vdev->irqlock, flags);
>> +
>> +	return ret;
>> +}
>> +
>>  static int vfio_intx_enable(struct vfio_pci_device *vdev)
>>  {
>>  	if (!is_irq_none(vdev))
>> @@ -208,7 +224,11 @@ static int vfio_intx_set_signal(struct vfio_pci_device *vdev, int fd)
>>  	if (!vdev->pci_2_3)
>>  		irqflags = 0;
>>  
>> -	ret = request_irq(pdev->irq, vfio_intx_handler,
>> +	if (vdev->ctx[0].deoi)
>> +		vdev->ctx[0].handler = vfio_intx_handler_deoi;
>> +	else
>> +		vdev->ctx[0].handler = vfio_intx_handler;
>> +	ret = request_irq(pdev->irq, vfio_intx_wrapper_handler,
>>  			  irqflags, vdev->ctx[0].name, vdev);
> 
> 
> Here's where I think we don't account for irqflags properly.  If we get
> a shared interrupt here, then enabling direct EOI needs to be disabled
> or else we'll starve other devices sharing the interrupt.  In practice,
> I wonder if this makes PCI direct EOI a useful feature.  We could try
> to get an exclusive interrupt and fallback to shared, but any time we
> get an exclusive interrupt we're more prone to conflicts with other
> devices.  I might have two VMs that share an interrupt and now it's a
> race that only the first to setup an IRQ can work.  Worse, one of those
> VMs might be fully booted and switched to MSI and now it's just a
> matter of time until they reboot in the right way to generate a
> conflict.  I might also have two devices in the same VM that share an
> IRQ and now I can't start the VM at all because the second device can
> no longer get an interrupt.  This is the same problem we have with the
> nointxmask flag, it's a useful debugging feature but since the masking
> is done at the APIC/GIC rather than the device, much like here, it's not
> very practical for more than debugging and isolating specific devices
> as requiring APIC/GIC level masking.  I'm not sure how to proceed on the
> PCI side here. Thanks,

So I agree Direct EOI with shared interrupts is a total mess as
- if the interrupt is not for VFIO, the physical interrupt will not be
deactivated
- if the interrupt is for VFIO, the physical interrupt will be
deactivated through guest virtual interrupt deactivation before
subsequent physical handlers complete their execution.

By the way, reading
"http://vfio.blogspot.fr/2014/09/vfio-interrupts-and-how-to-coax-windows.html"
was really helpful!

So I suggest I drop the feature for VFIO-PCI INTx and respin with
vfio-platform only. This series then mostly prepares for GICv4 integration.

Thanks

Eric


> 
> Alex
> 
>>  	if (ret) {
>>  		vdev->ctx[0].trigger = NULL;
>> diff --git a/drivers/vfio/pci/vfio_pci_private.h b/drivers/vfio/pci/vfio_pci_private.h
>> index f7f1101..5cfe59a 100644
>> --- a/drivers/vfio/pci/vfio_pci_private.h
>> +++ b/drivers/vfio/pci/vfio_pci_private.h
>> @@ -36,6 +36,8 @@ struct vfio_pci_irq_ctx {
>>  	char			*name;
>>  	bool			masked;
>>  	bool			automasked;
>> +	bool			deoi;
>> +	irqreturn_t		(*handler)(int irq, void *dev_id);
>>  	struct irq_bypass_producer	producer;
>>  };
>>  
>