Re: [PATCH V2 07/12] virtio-pci: harden INTX interrupts

[Jason Wang <jasowang@redhat.com>]

On Wed, Oct 13, 2021 at 5:42 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Tue, Oct 12, 2021 at 02:52:22PM +0800, Jason Wang wrote:
> > This patch tries to make sure the virtio interrupt handler for INTX
> > won't be called after a reset and before virtio_device_ready(). We
> > can't use IRQF_NO_AUTOEN since we're using shared interrupt
> > (IRQF_SHARED). So this patch tracks the INTX enabling status in a new
> > intx_soft_enabled variable and toggle it during in
> > vp_disable/enable_vectors(). The INTX interrupt handler will check
> > intx_soft_enabled before processing the actual interrupt.
> >
> > Cc: Boqun Feng <boqun.feng@gmail.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Paul E. McKenney <paulmck@kernel.org>
> > Signed-off-by: Jason Wang <jasowang@redhat.com>
> > ---
> >  drivers/virtio/virtio_pci_common.c | 24 ++++++++++++++++++++++--
> >  drivers/virtio/virtio_pci_common.h |  1 +
> >  2 files changed, 23 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/virtio/virtio_pci_common.c b/drivers/virtio/virtio_pci_common.c
> > index 0b9523e6dd39..5ae6a2a4eb77 100644
> > --- a/drivers/virtio/virtio_pci_common.c
> > +++ b/drivers/virtio/virtio_pci_common.c
> > @@ -30,8 +30,16 @@ void vp_disable_vectors(struct virtio_device *vdev)
> >       struct virtio_pci_device *vp_dev = to_vp_device(vdev);
> >       int i;
> >
> > -     if (vp_dev->intx_enabled)
> > +     if (vp_dev->intx_enabled) {
> > +             /*
> > +              * The below synchronize() guarantees that any
> > +              * interrupt for this line arriving after
> > +              * synchronize_irq() has completed is guaranteed to see
> > +              * intx_soft_enabled == false.
> > +              */
> > +             WRITE_ONCE(vp_dev->intx_soft_enabled, false);
> >               synchronize_irq(vp_dev->pci_dev->irq);
> > +     }
> >
> >       for (i = 0; i < vp_dev->msix_vectors; ++i)
> >               disable_irq(pci_irq_vector(vp_dev->pci_dev, i));
> > @@ -43,8 +51,16 @@ void vp_enable_vectors(struct virtio_device *vdev)
> >       struct virtio_pci_device *vp_dev = to_vp_device(vdev);
> >       int i;
> >
> > -     if (vp_dev->intx_enabled)
> > +     if (vp_dev->intx_enabled) {
> > +             disable_irq(vp_dev->pci_dev->irq);
> > +             /*
> > +              * The above disable_irq() provides TSO ordering and
> > +              * as such promotes the below store to store-release.
> > +              */
> > +             WRITE_ONCE(vp_dev->intx_soft_enabled, true);
> > +             enable_irq(vp_dev->pci_dev->irq);
> >               return;
> > +     }
> >
> >       for (i = 0; i < vp_dev->msix_vectors; ++i)
> >               enable_irq(pci_irq_vector(vp_dev->pci_dev, i));
> > @@ -97,6 +113,10 @@ static irqreturn_t vp_interrupt(int irq, void *opaque)
> >       struct virtio_pci_device *vp_dev = opaque;
> >       u8 isr;
> >
> > +     /* read intx_soft_enabled before read others */
> > +     if (!smp_load_acquire(&vp_dev->intx_soft_enabled))
> > +             return IRQ_NONE;
> > +
> >       /* reading the ISR has the effect of also clearing it so it's very
> >        * important to save off the value. */
> >       isr = ioread8(vp_dev->isr);
>
> I don't see why we need this ordering guarantee here.
>
> synchronize_irq above makes sure no interrupt handler
> is in progress.

Yes.

> the handler itself thus does not need
> any specific order, it is ok if intx_soft_enabled is read
> after, not before the rest of it.

But the interrupt could be raised after synchronize_irq() which may
see a false of the intx_soft_enabled. In this case we still need the
make sure intx_soft_enbled to be read first instead of allowing other
operations to be done first, otherwise the intx_soft_enabled is
meaningless.

Thanks

>
> Just READ_ONCE should be enough, and we can drop the comment.
>
>
> > diff --git a/drivers/virtio/virtio_pci_common.h b/drivers/virtio/virtio_pci_common.h
> > index a235ce9ff6a5..3c06e0f92ee4 100644
> > --- a/drivers/virtio/virtio_pci_common.h
> > +++ b/drivers/virtio/virtio_pci_common.h
> > @@ -64,6 +64,7 @@ struct virtio_pci_device {
> >       /* MSI-X support */
> >       int msix_enabled;
> >       int intx_enabled;
> > +     bool intx_soft_enabled;
> >       cpumask_var_t *msix_affinity_masks;
> >       /* Name strings for interrupts. This size should be enough,
> >        * and I'm too lazy to allocate each name separately. */
> > --
> > 2.25.1
>