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

Alex Williamson
2017-05-31T18:24:19Z

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,

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;
> };
>