Ladi Prosek
2016-Aug-31 12:00 UTC
[PATCH v2] virtio_ring: Make interrupt suppression spec compliant
According to the spec, if the VIRTIO_RING_F_EVENT_IDX feature bit is negotiated the driver MUST set flags to 0. Not dirtying the available ring in virtqueue_disable_cb also has a minor positive performance impact, improving L1 dcache load missed by ~0.5% in vring_bench. Writes to the used event field (vring_used_event) are still unconditional. Cc: Michael S. Tsirkin <mst at redhat.com> Cc: <stable at vger.kernel.org> # f277ec4 virtio_ring: shadow available Cc: <stable at vger.kernel.org> Signed-off-by: Ladi Prosek <lprosek at redhat.com> --- v1->v2: * fixed coding style * perf measurement results added to commit message * patch sent to virtualization at lists.linux-foundation.org (was kvm at vger.kernel.org) drivers/virtio/virtio_ring.c | 14 +++++++++----- 1 file changed, 9 insertions(+), 5 deletions(-) diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c index e383ecd..926ecb7 100644 --- a/drivers/virtio/virtio_ring.c +++ b/drivers/virtio/virtio_ring.c @@ -732,7 +732,8 @@ void virtqueue_disable_cb(struct virtqueue *_vq) if (!(vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)) { vq->avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT; - vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev, vq->avail_flags_shadow); + if (!vq->event) + vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev, vq->avail_flags_shadow); } } @@ -764,7 +765,8 @@ unsigned virtqueue_enable_cb_prepare(struct virtqueue *_vq) * entry. Always do both to keep code simple. */ if (vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT) { vq->avail_flags_shadow &= ~VRING_AVAIL_F_NO_INTERRUPT; - vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev, vq->avail_flags_shadow); + if (!vq->event) + vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev, vq->avail_flags_shadow); } vring_used_event(&vq->vring) = cpu_to_virtio16(_vq->vdev, last_used_idx = vq->last_used_idx); END_USE(vq); @@ -832,10 +834,11 @@ bool virtqueue_enable_cb_delayed(struct virtqueue *_vq) * more to do. */ /* Depending on the VIRTIO_RING_F_USED_EVENT_IDX feature, we need to * either clear the flags bit or point the event index at the next - * entry. Always do both to keep code simple. */ + * entry. Always update the event index to keep code simple. */ if (vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT) { vq->avail_flags_shadow &= ~VRING_AVAIL_F_NO_INTERRUPT; - vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev, vq->avail_flags_shadow); + if (!vq->event) + vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev, vq->avail_flags_shadow); } /* TODO: tune this threshold */ bufs = (u16)(vq->avail_idx_shadow - vq->last_used_idx) * 3 / 4; @@ -953,7 +956,8 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index, /* No callback? Tell other side not to bother us. */ if (!callback) { vq->avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT; - vq->vring.avail->flags = cpu_to_virtio16(vdev, vq->avail_flags_shadow); + if (!vq->event) + vq->vring.avail->flags = cpu_to_virtio16(vdev, vq->avail_flags_shadow); } /* Put everything in free lists. */ -- 2.5.5
Reasonably Related Threads
- [PATCH v2] virtio_ring: Make interrupt suppression spec compliant
- [PATCH] virtio_ring: Shadow available ring flags & index
- [RFC 3/3] virtio_ring: use new vring flags
- [PATCH] virtio_ring: Shadow available ring flags & index
- [PATCH] virtio_ring: Shadow available ring flags & index