Re: [RFC PATCH v6 15/20] vfio/platform: support for maskable and automasked interrupts

[Christoffer Dall <christoffer.dall@linaro.org>]

On Tue, Sep 02, 2014 at 06:06:17PM +0200, Antonios Motakis wrote:
> On Sun, Jun 8, 2014 at 12:17 PM, Christoffer Dall
> <christoffer.dall@linaro.org> wrote:
> > On Thu, Jun 05, 2014 at 07:03:23PM +0200, Antonios Motakis wrote:
> >> Adds support to mask interrupts, and also for automasked interrupts.
> >> Level sensitive interrupts are exposed as automasked interrupts and
> >> are masked and disabled automatically when they fire.
> >>
> >> Signed-off-by: Antonios Motakis <a.motakis@virtualopensystems.com>
> >> ---
> >>  drivers/vfio/platform/vfio_platform_irq.c     | 112 ++++++++++++++++++++++++--
> >>  drivers/vfio/platform/vfio_platform_private.h |   2 +
> >>  2 files changed, 109 insertions(+), 5 deletions(-)
> >>
> >> diff --git a/drivers/vfio/platform/vfio_platform_irq.c b/drivers/vfio/platform/vfio_platform_irq.c
> >> index d79f5af..10dfbf0 100644
> >> --- a/drivers/vfio/platform/vfio_platform_irq.c
> >> +++ b/drivers/vfio/platform/vfio_platform_irq.c
> >> @@ -51,9 +51,17 @@ int vfio_platform_irq_init(struct vfio_platform_device *vdev)
> >>               if (hwirq < 0)
> >>                       goto err;
> >>
> >> -             vdev->irq[i].flags = VFIO_IRQ_INFO_EVENTFD;
> >> +             spin_lock_init(&vdev->irq[i].lock);
> >> +
> >> +             vdev->irq[i].flags = VFIO_IRQ_INFO_EVENTFD
> >> +                                     | VFIO_IRQ_INFO_MASKABLE;
> >> +
> >> +             if (irq_get_trigger_type(hwirq) & IRQ_TYPE_LEVEL_MASK)
> >> +                     vdev->irq[i].flags |= VFIO_IRQ_INFO_AUTOMASKED;
> >
> > This seems to rely on the fact that you had actually loaded a driver for
> > your device to set the right type.  Is this assumption always correct?
> >
> > It seems to me that this configuration bit should now be up to your user
> > space drive who is the best candidate to know details about your device
> > at this point?
> >
> 
> Hm, I see this type being set usually either in a device tree source,
> or in the support code for a specific platform. Are there any
> situations where this is actually set by the driver? If I understand
> right this is not the case for the PL330, but if it is possible that
> it is the case for another device then I need to rethink this. Though
> as far as I understand this should not be the case.
> 

Wow, this has been incredibly long time since I looked at this code, so
not sure if I remember my original reasoning anymore, however,

while device properties are set in the DT, they would only be available
to this code if you actually loaded a device driver for that device,
right?  I'm just not sure that assumption always holds for devices used
by VFIO, but I'm really not sure anymore.  Maybe I'm rambling.

> >> +
> >>               vdev->irq[i].count = 1;
> >>               vdev->irq[i].hwirq = hwirq;
> >> +             vdev->irq[i].masked = false;
> >>       }
> >>
> >>       vdev->num_irqs = cnt;
> >> @@ -77,11 +85,27 @@ void vfio_platform_irq_cleanup(struct vfio_platform_device *vdev)
> >>
> >>  static irqreturn_t vfio_irq_handler(int irq, void *dev_id)
> >>  {
> >> -     struct eventfd_ctx *trigger = dev_id;
> >> +     struct vfio_platform_irq *irq_ctx = dev_id;
> >> +     unsigned long flags;
> >> +     int ret = IRQ_NONE;
> >> +
> >> +     spin_lock_irqsave(&irq_ctx->lock, flags);
> >> +
> >> +     if (!irq_ctx->masked) {
> >> +             ret = IRQ_HANDLED;
> >> +
> >> +             if (irq_ctx->flags & VFIO_IRQ_INFO_AUTOMASKED) {
> >> +                     disable_irq_nosync(irq_ctx->hwirq);
> >> +                     irq_ctx->masked = true;
> >> +             }
> >> +     }
> >>
> >> -     eventfd_signal(trigger, 1);
> >> +     spin_unlock_irqrestore(&irq_ctx->lock, flags);
> >>
> >> -     return IRQ_HANDLED;
> >> +     if (ret == IRQ_HANDLED)
> >> +             eventfd_signal(irq_ctx->trigger, 1);
> >> +
> >> +     return ret;
> >>  }
> >>
> >>  static int vfio_set_trigger(struct vfio_platform_device *vdev,
> >> @@ -162,6 +186,82 @@ static int vfio_platform_set_irq_trigger(struct vfio_platform_device *vdev,
> >>       return -EFAULT;
> >>  }
> >>
> >> +static int vfio_platform_set_irq_unmask(struct vfio_platform_device *vdev,
> >> +                                 unsigned index, unsigned start,
> >> +                                 unsigned count, uint32_t flags, void *data)
> >> +{
> >> +     uint8_t arr;
> >
> >
> > arr?
> 
> arr for array! As in, the VFIO API allows an array of IRQs. However
> for platform devices we don't use this, each IRQ is exposed
> independently as an array of 1 IRQ.
> 

but it's not an array.  If it contains IRQs, call it irqs.  Unless this
is referring specifically to a field *named* array, I don't remember the
API at current, but reading the code along it didn't make sense to me to
have a uint8_t called arr, and code should make as much sense
independenly as possible.

This reminds me of people writing code like:

String str;

yuck.

> >
> >> +
> >> +     if (start != 0 || count != 1)
> >> +             return -EINVAL;
> >> +
> >> +     switch (flags & VFIO_IRQ_SET_DATA_TYPE_MASK) {
> >> +     case VFIO_IRQ_SET_DATA_BOOL:
> >> +             if (copy_from_user(&arr, data, sizeof(uint8_t)))
> >> +                     return -EFAULT;
> >> +
> >> +             if (arr != 0x1)
> >> +                     return -EINVAL;
> >
> > why the fallthrough, what's this about?
> 
> The VFIO API allows to unmask/mask an array of IRQs, however with
> platform devices we only have arrays of 1 IRQ (so not really arrays).
> 
> So if the user uses VFIO_IRQ_SET_DATA_BOOL, we need to check that arr
> == 0x1. When that is the case, a fallthrough to the same code for
> VFIO_IRQ_SET_DATA_NONE is safe.
> 
> If that is not readable enough, then I can add a comment or duplicate
> the code that does the unmasking. I realize that if you don't know the
> VFIO API well, then this can look confusing.
> 

yeah, please put a big fat comment explaining the fallthrough.

-Christoffer