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SubjectRe: [PATCH v9 09/11] firmware: arm_scmi: Add atomic mode support to virtio transport
On Sun, Jan 23, 2022 at 05:40:08PM -0500, Michael S. Tsirkin wrote:
> 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 28
>
> Yea 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.
>

Thanks for your feedback !

I'll try to gather some perf data around it next days.
(and eventually cleanup and adding comments if it is god enough...)

Thanks,
Cristian

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