Re: [PATCH net-next RFC 1/3] virtio: support for urgent descriptors
From: Jason Wang
Date: Mon Oct 13 2014 - 02:22:46 EST
On 10/12/2014 05:27 PM, Michael S. Tsirkin wrote:
> On Sat, Oct 11, 2014 at 03:16:44PM +0800, Jason Wang wrote:
>> 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>
> 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.
>> 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(-)
>> +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;
> 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.
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