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

Auger Eric
2017-06-14T08:07:46Z

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;
>> };
>>
>