Subject: Re: [PATCH v9 09/11] firmware: arm_scmi: Add atomic mode support to virtio transport

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 23 Jan 2022 17:40:08 -0500

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.

-- 
MST

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