[PATCH 2/2] virtio_ring: return IRQ_HANDLED for stale interrupts when cb disabled

[Longjun Tang <lange_tang@163.com>]

From: Longjun Tang <tanglongjun@kylinos.cn>

In the vring_interrupt, if the used ring is empty, IRQ_NONE is returned.
However,Sometimes, such as with busy-polling, buffers might be consumed
from the used ring before an stale interrupt notification arrives. it
leading to return IRQ_NONE.

The kernel's spurious-IRQ detector counts consecutive IRQ_NONE returns
and will permanently disable the interrupt line if 99,900 out of 100,000
interrupts go unhandled.

Add is_cb_disabled() to virtqueue_ops and, when more_used() is false but
cb are suppressed, return IRQ_HANDLED instead of IRQ_NONE so the spurious
counter does not accumulate.

Signed-off-by: Longjun Tang <tanglongjun@kylinos.cn>
---
 drivers/virtio/virtio_ring.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 335692d41617..52df932fc4a2 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -185,6 +185,7 @@ struct virtqueue_ops {
 		     unsigned int last_used_idx);
 	void *(*detach_unused_buf)(struct vring_virtqueue *vq);
 	bool (*more_used)(const struct vring_virtqueue *vq);
+	bool (*is_cb_disabled)(const struct vring_virtqueue *vq);
 	int (*resize)(struct vring_virtqueue *vq, u32 num);
 	void (*reset)(struct vring_virtqueue *vq);
 };
@@ -1063,6 +1064,12 @@ static void virtqueue_disable_cb_split(struct vring_virtqueue *vq)
 	}
 }
 
+static bool is_cb_disabled_split(const struct vring_virtqueue *vq)
+{
+	return !!(data_race(vq->split.avail_flags_shadow) &
+		  VRING_AVAIL_F_NO_INTERRUPT);
+}
+
 static unsigned int virtqueue_enable_cb_prepare_split(struct vring_virtqueue *vq)
 {
 	u16 last_used_idx;
@@ -2227,6 +2234,12 @@ static void virtqueue_disable_cb_packed(struct vring_virtqueue *vq)
 	}
 }
 
+static bool is_cb_disabled_packed(const struct vring_virtqueue *vq)
+{
+	return data_race(vq->packed.event_flags_shadow) ==
+	       VRING_PACKED_EVENT_FLAG_DISABLE;
+}
+
 static unsigned int virtqueue_enable_cb_prepare_packed(struct vring_virtqueue *vq)
 {
 	START_USE(vq);
@@ -2644,6 +2657,7 @@ static const struct virtqueue_ops split_ops = {
 	.poll = virtqueue_poll_split,
 	.detach_unused_buf = virtqueue_detach_unused_buf_split,
 	.more_used = more_used_split,
+	.is_cb_disabled = is_cb_disabled_split,
 	.resize = virtqueue_resize_split,
 	.reset = virtqueue_reset_split,
 };
@@ -2658,6 +2672,7 @@ static const struct virtqueue_ops packed_ops = {
 	.poll = virtqueue_poll_packed,
 	.detach_unused_buf = virtqueue_detach_unused_buf_packed,
 	.more_used = more_used_packed,
+	.is_cb_disabled = is_cb_disabled_packed,
 	.resize = virtqueue_resize_packed,
 	.reset = virtqueue_reset_packed,
 };
@@ -2672,6 +2687,7 @@ static const struct virtqueue_ops split_in_order_ops = {
 	.poll = virtqueue_poll_split,
 	.detach_unused_buf = virtqueue_detach_unused_buf_split,
 	.more_used = more_used_split_in_order,
+	.is_cb_disabled = is_cb_disabled_split,
 	.resize = virtqueue_resize_split,
 	.reset = virtqueue_reset_split,
 };
@@ -2686,6 +2702,7 @@ static const struct virtqueue_ops packed_in_order_ops = {
 	.poll = virtqueue_poll_packed,
 	.detach_unused_buf = virtqueue_detach_unused_buf_packed,
 	.more_used = more_used_packed_in_order,
+	.is_cb_disabled = is_cb_disabled_packed,
 	.resize = virtqueue_resize_packed,
 	.reset = virtqueue_reset_packed,
 };
@@ -3231,6 +3248,18 @@ irqreturn_t vring_interrupt(int irq, void *_vq)
 	struct vring_virtqueue *vq = to_vvq(_vq);
 
 	if (!more_used(vq)) {
+		/*
+		 * Stale interrupt: the device posted this notification
+		 * before it observed the callback suppression;
+		 * When more_used returns empty, IRQ_HANDLED should be
+		 * returned for stale interrupts.
+		 */
+		if (VIRTQUEUE_CALL(vq, is_cb_disabled)) {
+			if (vq->event)
+				data_race(vq->event_triggered = true);
+			pr_debug("virtqueue stale interrupt (callbacks disabled) for %p\n", vq);
+			return IRQ_HANDLED;
+		}
 		pr_debug("virtqueue interrupt with no work for %p\n", vq);
 		return IRQ_NONE;
 	}
-- 
2.43.0