Re: [PATCH 7/9] virtio-pci: harden INTX interrupts

[Jason Wang]

On Mon, Sep 13, 2021 at 2:33 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> On Mon, Sep 13, 2021 at 01:53:51PM +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.
> >
> > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
>
>
> Not all that excited about all the memory barriers for something
> that should be an extremely rare event (for most kernels -
> literally once per boot). Can't we do better?

I'm not sure, but do we need to care about the slow path (INTX)?

(Or do you have a better approach?)

Thanks

>
> > ---
> >  drivers/virtio/virtio_pci_common.c | 18 ++++++++++++++++--
> >  drivers/virtio/virtio_pci_common.h |  1 +
> >  2 files changed, 17 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/virtio/virtio_pci_common.c b/drivers/virtio/virtio_pci_common.c
> > index 0b9523e6dd39..835197151dc1 100644
> > --- a/drivers/virtio/virtio_pci_common.c
> > +++ b/drivers/virtio/virtio_pci_common.c
> > @@ -30,8 +30,12 @@ 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) {
> > +             vp_dev->intx_soft_enabled = false;
> > +             /* ensure the vp_interrupt see this intx_soft_enabled value */
> > +             smp_wmb();
> >               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 +47,12 @@ 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) {
> > +             vp_dev->intx_soft_enabled = true;
> > +             /* ensure the vp_interrupt see this intx_soft_enabled value */
> > +             smp_wmb();
> >               return;
> > +     }
> >
> >       for (i = 0; i < vp_dev->msix_vectors; ++i)
> >               enable_irq(pci_irq_vector(vp_dev->pci_dev, i));
> > @@ -97,6 +105,12 @@ static irqreturn_t vp_interrupt(int irq, void *opaque)
> >       struct virtio_pci_device *vp_dev = opaque;
> >       u8 isr;
> >
> > +     if (!vp_dev->intx_soft_enabled)
> > +             return IRQ_NONE;
> > +
> > +     /* read intx_soft_enabled before read others */
> > +     smp_rmb();
> > +
> >       /* 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);
> > 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
>