Michael S. Tsirkin
2022-Jan-23 22:40 UTC
[PATCH v9 09/11] firmware: arm_scmi: Add atomic mode support to virtio transport
On Sun, Jan 23, 2022 at 08:02:54PM +0000, Cristian Marussi wrote:> I was thinking...keeping the current virtqueue_poll interface, since our > possible issue arises from the used_index wrapping around exactly on top > of the same polled index and given that currently the API returns an > unsigned "opaque" value really carrying just the 16-bit index (and possibly > the wrap bit as bit15 for packed vq) that is supposed to be fed back as > it is to the virtqueue_poll() function.... > > ...why don't we just keep an internal full fledged per-virtqueue wrap-counter > and return that as the MSB 16-bit of the opaque value returned by > virtqueue_prepare_enable_cb and then check it back in virtqueue_poll when the > opaque is fed back ? (filtering it out from the internal helpers machinery) > > As in the example below the scissors. > > I mean if the internal wrap count is at that point different from the > one provided to virtqueue_poll() via the opaque poll_idx value previously > provided, certainly there is something new to fetch without even looking > at the indexes: at the same time, exposing an opaque index built as > (wraps << 16 | idx) implicitly 'binds' each index to a specific > wrap-iteration, so they can be distiguished (..ok until the wrap-count > upper 16bit wraps too....but...) > > I am not really extremely familiar with the internals of virtio so I > could be missing something obvious...feel free to insult me :P > > (..and I have not made any perf measurements or consideration at this > point....nor considered the redundancy of the existent packed > used_wrap_counter bit...) > > Thanks, > Cristian > > ---- > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c > index 00f64f2f8b72..bda6af121cd7 100644 > --- a/drivers/virtio/virtio_ring.c > +++ b/drivers/virtio/virtio_ring.c > @@ -117,6 +117,8 @@ struct vring_virtqueue { > /* Last used index we've seen. */ > u16 last_used_idx; > > + u16 wraps; > + > /* Hint for event idx: already triggered no need to disable. */ > bool event_triggered; > > @@ -806,6 +808,8 @@ static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq, > ret = vq->split.desc_state[i].data; > detach_buf_split(vq, i, ctx); > vq->last_used_idx++; > + if (unlikely(!vq->last_used_idx)) > + vq->wraps++;I wonder whether vq->wraps += !vq->last_used_idx; is faster or slower. No branch but OTOH a dependency.> /* If we expect an interrupt for the next entry, tell host > * by writing event index and flush out the write before > * the read in the next get_buf call. */ > @@ -1508,6 +1512,7 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq, > if (unlikely(vq->last_used_idx >= vq->packed.vring.num)) { > vq->last_used_idx -= vq->packed.vring.num; > vq->packed.used_wrap_counter ^= 1; > + vq->wraps++; > } > > /* > @@ -1744,6 +1749,7 @@ static struct virtqueue *vring_create_virtqueue_packed( > vq->weak_barriers = weak_barriers; > vq->broken = false; > vq->last_used_idx = 0; > + vq->wraps = 0; > vq->event_triggered = false; > vq->num_added = 0; > vq->packed_ring = true; > @@ -2092,13 +2098,17 @@ EXPORT_SYMBOL_GPL(virtqueue_disable_cb); > */ > unsigned virtqueue_enable_cb_prepare(struct virtqueue *_vq) > { > + unsigned last_used_idx; > struct vring_virtqueue *vq = to_vvq(_vq); > > if (vq->event_triggered) > vq->event_triggered = false; > > - return vq->packed_ring ? virtqueue_enable_cb_prepare_packed(_vq) : > - virtqueue_enable_cb_prepare_split(_vq); > + last_used_idx = vq->packed_ring ? > + virtqueue_enable_cb_prepare_packed(_vq) : > + virtqueue_enable_cb_prepare_split(_vq); > + > + return VRING_BUILD_OPAQUE(last_used_idx, vq->wraps); > } > EXPORT_SYMBOL_GPL(virtqueue_enable_cb_prepare); > > @@ -2118,9 +2128,13 @@ bool virtqueue_poll(struct virtqueue *_vq, unsigned last_used_idx) > if (unlikely(vq->broken)) > return false; > > + if (unlikely(vq->wraps != VRING_GET_WRAPS(last_used_idx))) > + return true; > + > virtio_mb(vq->weak_barriers); > - return vq->packed_ring ? virtqueue_poll_packed(_vq, last_used_idx) : > - virtqueue_poll_split(_vq, last_used_idx); > + return vq->packed_ring ? > + virtqueue_poll_packed(_vq, VRING_GET_IDX(last_used_idx)) : > + virtqueue_poll_split(_vq, VRING_GET_IDX(last_used_idx)); > } > EXPORT_SYMBOL_GPL(virtqueue_poll); > > @@ -2245,6 +2259,7 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index, > vq->weak_barriers = weak_barriers; > vq->broken = false; > vq->last_used_idx = 0; > + vq->wraps = 0; > vq->event_triggered = false; > vq->num_added = 0; > vq->use_dma_api = vring_use_dma_api(vdev); > diff --git a/include/uapi/linux/virtio_ring.h b/include/uapi/linux/virtio_ring.h > index 476d3e5c0fe7..e6b03017ebd7 100644 > --- a/include/uapi/linux/virtio_ring.h > +++ b/include/uapi/linux/virtio_ring.h > @@ -77,6 +77,17 @@ > */ > #define VRING_PACKED_EVENT_F_WRAP_CTR 15 > > +#define VRING_IDX_MASK GENMASK(15, 0) > +#define VRING_GET_IDX(opaque) \ > + ((u16)FIELD_GET(VRING_IDX_MASK, (opaque))) > + > +#define VRING_WRAPS_MASK GENMASK(31, 16) > +#define VRING_GET_WRAPS(opaque) \ > + ((u16)FIELD_GET(VRING_WRAPS_MASK, (opaque))) > + > +#define VRING_BUILD_OPAQUE(idx, wraps) \ > + (FIELD_PREP(VRING_WRAPS_MASK, (wraps)) | ((idx) & VRING_IDX_MASK)) > + > /* We support indirect buffer descriptors */ > #define VIRTIO_RING_F_INDIRECT_DESC 28Yea I think this patch increases the time it takes to wrap around from 2^16 to 2^32 which seems good enough. Need some comments to explain the logic. Would be interesting to see perf data. -- MST