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

From: Rusty Russell
Date: Wed Oct 15 2014 - 02:34:25 EST

Jason Wang <jasowang@xxxxxxxxxx> writes:
> 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@xxxxxxxxxx>
> Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>

The new VRING_DESC_F_URGENT bit is theoretically nicer, but for
networking (which tends to take packets in order) couldn't we just set
the event counter to give us a tx interrupt at the packet we want?

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