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vhost: drop VIRTIO_F_NOTIFY_ON_EMPTY feature

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This feature is not particularly useful for storage.
It forces us to read additional vq memory on each
I/O completion and that's quite expensive.

Quoting VIRTIO 1.0:

```
VIRTIO_F_NOTIFY_ON_EMPTY (24)
If this feature has been negotiated by driver, the device MUST issue an
interrupt if the device runs out of available descriptors on a
virtqueue, even though interrupts are suppressed using the
VIRTQ_AVAIL_F_NO_INTERRUPT flag or the used_event field.
```

Later on:

```
Note: An example of a driver using this feature is the legacy networking
driver: it doesn’t need to know every time a packet is transmitted, but
it does need to free the transmitted packets a finite time after they
are transmitted. It can avoid using a timer if the device interrupts it
when all the packets are transmitted.
```

Change-Id: I7f53293bf811a4cd5ae8e42e18f35042ea6f4ba8
Suggested-by: Pawel Wodkowski <pawelx.wodkowski@intel.com>
Signed-off-by: Dariusz Stojaczyk <dariuszx.stojaczyk@intel.com>
Reviewed-on: https://review.gerrithub.io/398325
Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Pawel Wodkowski <pawelx.wodkowski@intel.com>
Reviewed-by: Daniel Verkamp <daniel.verkamp@intel.com>
This commit is contained in:
Dariusz Stojaczyk 2018-02-04 13:24:13 +01:00 committed by Jim Harris
parent d03cac1310
commit da78c67091
2 changed files with 2 additions and 6 deletions
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5
lib/vhost/vhost.c
View File

@ -374,11 +374,6 @@ spdk_vhost_vq_used_ring_enqueue(struct spdk_vhost_dev *vdev, struct spdk_vhost_v
spdk_wmb();
virtqueue->used_req_cnt++;
if (spdk_vhost_dev_has_feature(vdev, VIRTIO_F_NOTIFY_ON_EMPTY) &&
spdk_unlikely(vring->avail->idx == vring->last_avail_idx)) {
spdk_vhost_vq_used_signal(vdev, virtqueue);
}
}
int

3
lib/vhost/vhost_internal.h
View File

@ -91,7 +91,8 @@
(1ULL << VIRTIO_RING_F_INDIRECT_DESC))
#define SPDK_VHOST_DISABLED_FEATURES ((1ULL << VIRTIO_RING_F_EVENT_IDX) | \
(1ULL << VIRTIO_RING_F_INDIRECT_DESC))
(1ULL << VIRTIO_RING_F_INDIRECT_DESC) | \
(1ULL << VIRTIO_F_NOTIFY_ON_EMPTY))
struct spdk_vhost_virtqueue {
struct rte_vhost_vring vring;