Re: [PATCH net-next RFC 1/3] virtio: support for urgent descriptors

On Mon, Oct 13, 2014 at 02:22:12PM +0800, Jason Wang wrote:

On 10/12/2014 05:27 PM, Michael S. Tsirkin wrote:

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On Sat, Oct 11, 2014 at 03:16:44PM +0800, Jason Wang wrote:

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Below should be useful for some experiments Jason is doing.
I thought I'd send it out for early review/feedback.

event idx feature allows us to defer interrupts until
a specific # of descriptors were used.
Sometimes it might be useful to get an interrupt after
a specific descriptor, regardless.
This adds a descriptor flag for this, and an API
to create an urgent output descriptor.
This is still an RFC:
we'll need a feature bit for drivers to detect this,
but we've run out of feature bits for virtio 0.X.
For experimentation purposes, drivers can assume
this is set, or add a driver-specific feature bit.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <redacted>

I see that as compared to my original patch, you have
added a new flag: VRING_AVAIL_F_NO_URGENT_INTERRUPT
I don't think it's necessary, see below.

As such, I think this patch should be split:
- original patch adding support for urgent descriptors
- a patch adding virtqueue_enable/disable_cb_urgent(_prepare)?

Not sure this is a good idea, since the api of first patch is in-completed.

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---
 drivers/virtio/virtio_ring.c     | 75 +++++++++++++++++++++++++++++++++++++---
 include/linux/virtio.h           | 14 ++++++++
 include/uapi/linux/virtio_ring.h |  5 ++-
 3 files changed, 89 insertions(+), 5 deletions(-)

[...]

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+unsigned virtqueue_enable_cb_prepare_urgent(struct virtqueue *_vq)
+{
+	struct vring_virtqueue *vq = to_vvq(_vq);
+	u16 last_used_idx;
+
+	START_USE(vq);
+	vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_URGENT_INTERRUPT;
+	last_used_idx = vq->last_used_idx;
+	END_USE(vq);
+	return last_used_idx;
+}
+EXPORT_SYMBOL_GPL(virtqueue_enable_cb_prepare_urgent);
+

You can implement virtqueue_enable_cb_prepare_urgent
simply by clearing ~VRING_AVAIL_F_NO_INTERRUPT.

The effect is same: host sends interrupts only if there
is an urgent descriptor.

Seems not, consider the case when event index was disabled. This will
turn on all interrupts.

This means that a legacy device without event index support
will get more interrupts.
Sounds reasonable.

-- 
MST