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Date: Sun, 23 Jan 2022 17:40:08 -0500
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Cristian Marussi <cristian.marussi@....com>
Cc: Peter Hilber <peter.hilber@...nsynergy.com>,
linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
sudeep.holla@....com, james.quinlan@...adcom.com,
Jonathan.Cameron@...wei.com, f.fainelli@...il.com,
etienne.carriere@...aro.org, vincent.guittot@...aro.org,
souvik.chakravarty@....com, igor.skalkin@...nsynergy.com,
virtualization@...ts.linux-foundation.org
Subject: Re: [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 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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