On Thu, Oct 14, 2021 at 10:35:48AM +0800, Jason Wang wrote:

On Wed, Oct 13, 2021 at 5:42 PM Michael S. Tsirkin [off-list ref] wrote:

quoted

On Tue, Oct 12, 2021 at 02:52:22PM +0800, Jason Wang wrote:

quoted

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 <redacted>
Cc: Thomas Gleixner <redacted>
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.

quoted

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.

You mean a "true" value right? false is what we are writing there.

Are you sure it can happen? I think that synchronize_irq makes the value
visible on all CPUs running the irq.

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

If intx_soft_enbled were not visible after synchronize_irq then
it does not matter in which order we read it wrt other values,
it still wouldn't work right.

quoted

Just READ_ONCE should be enough, and we can drop the comment.

quoted

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

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