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Date:   Tue, 17 Oct 2023 15:28:05 +0200
From:   Stefano Garzarella <sgarzare@redhat.com>
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Matias Ezequiel Vara Larsen <mvaralar@redhat.com>,
        anton.yakovlev@opensynergy.com, perex@perex.cz, tiwai@suse.com,
        virtualization@lists.linux-foundation.org,
        alsa-devel@alsa-project.org, linux-kernel@vger.kernel.org,
        pbonzini@redhat.com, stefanha@redhat.com,
        manos.pitsidianakis@linaro.org, mripard@redhat.com
Subject: Re: [RFC PATCH] ALSA: virtio: use copy and fill_silence callbacks
Message-ID: <g7hi2yqqiee5tyehuyn422uzxz6tc3kfbahgjaykrnpm43zisn@prfjfy7isuv7>
References: <ZSgMeoMx6NX2zCx/@fedora>
 <20231012111525-mutt-send-email-mst@kernel.org>
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On Thu, Oct 12, 2023 at 11:16:54AM -0400, Michael S. Tsirkin wrote:
>On Thu, Oct 12, 2023 at 05:10:50PM +0200, Matias Ezequiel Vara Larsen wrote:
>> This commit replaces the mmap mechanism with the copy() and
>> fill_silence() callbacks for both capturing and playback for the
>> virtio-sound driver. This change is required to prevent the updating of
>> the content of a buffer that is already in the available ring.
>>
>> The current mechanism splits a dma buffer into descriptors that are
>> exposed to the device. This dma buffer is shared with the user
>> application. When the device consumes a buffer, the driver moves the
>> request from the used ring to available ring.
>>
>> The driver exposes the buffer to the device without knowing if the
>> content has been updated from the user. The section 2.8.21.1 of the
>> virtio spec states that: "The device MAY access the descriptor chains
>> the driver created and the memory they refer to immediately". If the
>> device picks up buffers from the available ring just after it is
>> notified, it happens that the content may be old.
>>
>> By providing the copy() callback, the driver first updates the content
>> of the buffer, and then, exposes the buffer to the device by enqueuing
>> it in the available ring. Thus, device always picks up a buffer that is
>> updated.
>>
>> For capturing, the driver starts by exposing all the available buffers
>> to device. After device updates the content of a buffer, it enqueues it
>> in the used ring. It is only after the copy() for capturing is issued
>> that the driver re-enqueues the buffer in the available ring.
>>
>> Note that the copy() function assumes that user is always writing a
>> period. Testing shows that this is true but I may be wrong. This RFC
>> aims at clarifying this.
>>
>> Signed-off-by: Matias Ezequiel Vara Larsen <mvaralar@redhat.com>
>
>
>Thank you for working on this!

Yep, +1!

@Michael do you think we should cc stable and add a Fixes tag since
the driver is not following the virtio spec?

Or it is too risky?

IIUC snd_pcm_ops is changed a bit from previous versions, so we may have
to adapt the patch for stable branches.

Stefano

>
>> ---
>>  sound/virtio/virtio_pcm.c     | 11 ++--
>>  sound/virtio/virtio_pcm.h     |  9 +++-
>>  sound/virtio/virtio_pcm_msg.c | 50 ++++++++++++++++---
>>  sound/virtio/virtio_pcm_ops.c | 94 +++++++++++++++++++++++++++++++----
>>  4 files changed, 137 insertions(+), 27 deletions(-)
>>
>> diff --git a/sound/virtio/virtio_pcm.c b/sound/virtio/virtio_pcm.c
>> index c10d91fff2fb..bfe982952303 100644
>> --- a/sound/virtio/virtio_pcm.c
>> +++ b/sound/virtio/virtio_pcm.c
>> @@ -104,8 +104,6 @@ static int virtsnd_pcm_build_hw(struct virtio_pcm_substream *vss,
>>  	 * only message-based transport.
>>  	 */
>>  	vss->hw.info =
>> -		SNDRV_PCM_INFO_MMAP |
>> -		SNDRV_PCM_INFO_MMAP_VALID |
>>  		SNDRV_PCM_INFO_BATCH |
>>  		SNDRV_PCM_INFO_BLOCK_TRANSFER |
>>  		SNDRV_PCM_INFO_INTERLEAVED |
>> @@ -471,12 +469,11 @@ int virtsnd_pcm_build_devs(struct virtio_snd *snd)
>>  			for (kss = ks->substream; kss; kss = kss->next)
>>  				vs->substreams[kss->number]->substream = kss;
>>
>> -			snd_pcm_set_ops(vpcm->pcm, i, &virtsnd_pcm_ops);
>> +			if (i == SNDRV_PCM_STREAM_CAPTURE)
>> +				snd_pcm_set_ops(vpcm->pcm, i, &virtsnd_pcm_capture_ops);
>> +			else
>> +				snd_pcm_set_ops(vpcm->pcm, i, &virtsnd_pcm_playback_ops);
>>  		}
>> -
>> -		snd_pcm_set_managed_buffer_all(vpcm->pcm,
>> -					       SNDRV_DMA_TYPE_VMALLOC, NULL,
>> -					       0, 0);
>>  	}
>>
>>  	return 0;
>> diff --git a/sound/virtio/virtio_pcm.h b/sound/virtio/virtio_pcm.h
>> index 062eb8e8f2cf..1c1106ec971f 100644
>> --- a/sound/virtio/virtio_pcm.h
>> +++ b/sound/virtio/virtio_pcm.h
>> @@ -50,6 +50,8 @@ struct virtio_pcm_substream {
>>  	struct work_struct elapsed_period;
>>  	spinlock_t lock;
>>  	size_t buffer_bytes;
>> +	u8 *buffer;
>> +	size_t buffer_sz;
>>  	size_t hw_ptr;
>>  	bool xfer_enabled;
>>  	bool xfer_xrun;
>> @@ -90,7 +92,8 @@ struct virtio_pcm {
>>  	struct virtio_pcm_stream streams[SNDRV_PCM_STREAM_LAST + 1];
>>  };
>>
>> -extern const struct snd_pcm_ops virtsnd_pcm_ops;
>> +extern const struct snd_pcm_ops virtsnd_pcm_playback_ops;
>> +extern const struct snd_pcm_ops virtsnd_pcm_capture_ops;
>>
>>  int virtsnd_pcm_validate(struct virtio_device *vdev);
>>
>> @@ -117,7 +120,9 @@ int virtsnd_pcm_msg_alloc(struct virtio_pcm_substream *vss,
>>
>>  void virtsnd_pcm_msg_free(struct virtio_pcm_substream *vss);
>>
>> -int virtsnd_pcm_msg_send(struct virtio_pcm_substream *vss);
>> +int virtsnd_pcm_msg_send(struct virtio_pcm_substream *vss, bool single);
>> +
>> +int virtsnd_pcm_msg_send_locked(struct virtio_pcm_substream *vss, bool single);
>>
>>  unsigned int virtsnd_pcm_msg_pending_num(struct virtio_pcm_substream *vss);
>>
>> diff --git a/sound/virtio/virtio_pcm_msg.c b/sound/virtio/virtio_pcm_msg.c
>> index aca2dc1989ba..9a5f9814cb62 100644
>> --- a/sound/virtio/virtio_pcm_msg.c
>> +++ b/sound/virtio/virtio_pcm_msg.c
>> @@ -132,7 +132,6 @@ static void virtsnd_pcm_sg_from(struct scatterlist *sgs, int nsgs, u8 *data,
>>  int virtsnd_pcm_msg_alloc(struct virtio_pcm_substream *vss,
>>  			  unsigned int periods, unsigned int period_bytes)
>>  {
>> -	struct snd_pcm_runtime *runtime = vss->substream->runtime;
>>  	unsigned int i;
>>
>>  	vss->msgs = kcalloc(periods, sizeof(*vss->msgs), GFP_KERNEL);
>> @@ -142,7 +141,7 @@ int virtsnd_pcm_msg_alloc(struct virtio_pcm_substream *vss,
>>  	vss->nmsgs = periods;
>>
>>  	for (i = 0; i < periods; ++i) {
>> -		u8 *data = runtime->dma_area + period_bytes * i;
>> +		u8 *data = vss->buffer + period_bytes * i;
>>  		int sg_num = virtsnd_pcm_sg_num(data, period_bytes);
>>  		struct virtio_pcm_msg *msg;
>>
>> @@ -186,10 +185,12 @@ void virtsnd_pcm_msg_free(struct virtio_pcm_substream *vss)
>>  /**
>>   * virtsnd_pcm_msg_send() - Send asynchronous I/O messages.
>>   * @vss: VirtIO PCM substream.
>> + * @single: true to enqueue a single message, false to enqueue all of them.
>>   *
>>   * All messages are organized in an ordered circular list. Each time the
>> - * function is called, all currently non-enqueued messages are added to the
>> - * virtqueue. For this, the function keeps track of two values:
>> + * function is called, first non-enqueued message is added to the virtqueue.
>> + * When single is True, only the first message is enqueued. When False, all the
>> + * available messages are enqueued.  The function keeps track of two values:
>>   *
>>   *   msg_last_enqueued = index of the last enqueued message,
>>   *   msg_count = # of pending messages in the virtqueue.
>> @@ -198,7 +199,7 @@ void virtsnd_pcm_msg_free(struct virtio_pcm_substream *vss)
>>   *          spinlocks to be held by caller.
>>   * Return: 0 on success, -errno on failure.
>>   */
>> -int virtsnd_pcm_msg_send(struct virtio_pcm_substream *vss)
>> +int virtsnd_pcm_msg_send(struct virtio_pcm_substream *vss, bool single)
>>  {
>>  	struct snd_pcm_runtime *runtime = vss->substream->runtime;
>>  	struct virtio_snd *snd = vss->snd;
>> @@ -211,6 +212,13 @@ int virtsnd_pcm_msg_send(struct virtio_pcm_substream *vss)
>>  	i = (vss->msg_last_enqueued + 1) % runtime->periods;
>>  	n = runtime->periods - vss->msg_count;
>>
>> +	if (single) {
>> +		if (n < 1)
>> +			return -EFAULT;
>> +
>> +		n = 1;
>> +	}
>> +
>>  	for (; n; --n, i = (i + 1) % runtime->periods) {
>>  		struct virtio_pcm_msg *msg = vss->msgs[i];
>>  		struct scatterlist *psgs[] = {
>> @@ -250,6 +258,36 @@ int virtsnd_pcm_msg_send(struct virtio_pcm_substream *vss)
>>  	return 0;
>>  }
>>
>> +/**
>> + * virtsnd_pcm_msg_send_locked() - Send asynchronous I/O messages.
>> + * @vss: VirtIO PCM substream.
>> + * @single: true to enqueue a single message, false to enqueue all of them.
>> + *
>> + * This function holds the tx/rx queue and the VirtIO substream spinlocks
>> + * before calling virtsnd_pcm_msg_send(). This is a wrapper function to ease
>> + * the invocation of virtsnd_pcm_msg_send().
>> + *
>> + * Context: Any context.
>> + * Return: 0 on success, -errno on failure.
>> + */
>> +
>> +int virtsnd_pcm_msg_send_locked(struct virtio_pcm_substream *vss, bool single)
>> +{
>> +	struct virtio_snd_queue *queue;
>> +	int rc;
>> +	unsigned long flags;
>> +
>> +	queue = virtsnd_pcm_queue(vss);
>> +
>> +	spin_lock_irqsave(&queue->lock, flags);
>> +	spin_lock(&vss->lock);
>> +	rc = virtsnd_pcm_msg_send(vss, single);
>> +	spin_unlock(&vss->lock);
>> +	spin_unlock_irqrestore(&queue->lock, flags);
>> +
>> +	return rc;
>> +}
>> +
>>  /**
>>   * virtsnd_pcm_msg_pending_num() - Returns the number of pending I/O messages.
>>   * @vss: VirtIO substream.
>> @@ -320,8 +358,6 @@ static void virtsnd_pcm_msg_complete(struct virtio_pcm_msg *msg,
>>  					le32_to_cpu(msg->status.latency_bytes));
>>
>>  		schedule_work(&vss->elapsed_period);
>> -
>> -		virtsnd_pcm_msg_send(vss);
>>  	} else if (!vss->msg_count) {
>>  		wake_up_all(&vss->msg_empty);
>>  	}
>> diff --git a/sound/virtio/virtio_pcm_ops.c b/sound/virtio/virtio_pcm_ops.c
>> index f8bfb87624be..a208439dbff8 100644
>> --- a/sound/virtio/virtio_pcm_ops.c
>> +++ b/sound/virtio/virtio_pcm_ops.c
>> @@ -238,6 +238,11 @@ static int virtsnd_pcm_hw_params(struct snd_pcm_substream *substream,
>>  	 */
>>  	virtsnd_pcm_msg_free(vss);
>>
>> +	vss->buffer_sz = params_buffer_bytes(hw_params);
>> +	vss->buffer = alloc_pages_exact(vss->buffer_sz, GFP_KERNEL);
>> +	if (!vss->buffer)
>> +		return -ENOMEM;
>> +
>>  	return virtsnd_pcm_msg_alloc(vss, params_periods(hw_params),
>>  				     params_period_bytes(hw_params));
>>  }
>> @@ -257,6 +262,11 @@ static int virtsnd_pcm_hw_free(struct snd_pcm_substream *substream)
>>  	if (!virtsnd_pcm_msg_pending_num(vss))
>>  		virtsnd_pcm_msg_free(vss);
>>
>> +	if (vss->buffer) {
>> +		free_pages_exact(vss->buffer, vss->buffer_sz);
>> +		vss->buffer = NULL;
>> +	}
>> +
>>  	return 0;
>>  }
>>
>> @@ -331,15 +341,18 @@ static int virtsnd_pcm_trigger(struct snd_pcm_substream *substream, int command)
>>  	case SNDRV_PCM_TRIGGER_PAUSE_RELEASE:
>>  		queue = virtsnd_pcm_queue(vss);
>>
>> -		spin_lock_irqsave(&queue->lock, flags);
>> -		spin_lock(&vss->lock);
>> -		rc = virtsnd_pcm_msg_send(vss);
>> -		if (!rc)
>> -			vss->xfer_enabled = true;
>> -		spin_unlock(&vss->lock);
>> -		spin_unlock_irqrestore(&queue->lock, flags);
>> -		if (rc)
>> -			return rc;
>> +		// The buffers should be exposed first during capturing so that
>> +		// the device can consume them. Capturing cannot begin
>> +		// otherwise.
>> +		if (vss->direction == SNDRV_PCM_STREAM_CAPTURE) {
>> +			rc = virtsnd_pcm_msg_send_locked(vss, false);
>> +			if (rc)
>> +				return rc;
>> +		}
>> +
>> +		spin_lock_irqsave(&vss->lock, flags);
>> +		vss->xfer_enabled = true;
>> +		spin_unlock_irqrestore(&vss->lock, flags);
>>
>>  		msg = virtsnd_pcm_ctl_msg_alloc(vss, VIRTIO_SND_R_PCM_START,
>>  						GFP_KERNEL);
>> @@ -450,8 +463,66 @@ virtsnd_pcm_pointer(struct snd_pcm_substream *substream)
>>  	return hw_ptr;
>>  }
>>
>> -/* PCM substream operators map. */
>> -const struct snd_pcm_ops virtsnd_pcm_ops = {
>> +static int virtsnd_pcm_pb_copy(struct snd_pcm_substream *substream,
>> +			       int channel, unsigned long pos, struct iov_iter
>> +			       *src, unsigned long count)
>> +{
>> +	struct virtio_pcm_substream *vss = snd_pcm_substream_chip(substream);
>> +
>> +	if (unlikely(pos + count > vss->buffer_sz))
>> +		return -EINVAL;
>> +
>> +	if (copy_from_iter(vss->buffer + pos, count, src) != count)
>> +		return -EFAULT;
>> +
>> +	return virtsnd_pcm_msg_send_locked(vss, true);
>> +}
>> +
>> +static int virtsnd_pcm_cap_copy(struct snd_pcm_substream *substream,
>> +				int channel, unsigned long pos, struct iov_iter
>> +				*dst, unsigned long count)
>> +{
>> +	struct virtio_pcm_substream *vss = snd_pcm_substream_chip(substream);
>> +
>> +	if (unlikely(pos + count > vss->buffer_sz))
>> +		return -EINVAL;
>> +
>> +	if (copy_to_iter(vss->buffer + pos, count, dst) != count)
>> +		return -EFAULT;
>> +
>> +	return virtsnd_pcm_msg_send_locked(vss, true);
>> +}
>> +
>> +static int virtsnd_pcm_pb_silence(struct snd_pcm_substream *substream, int channel,
>> +				  unsigned long pos, unsigned long count)
>> +{
>> +	struct virtio_pcm_substream *vss = snd_pcm_substream_chip(substream);
>> +
>> +	if (unlikely(pos + count > vss->buffer_sz))
>> +		return -EINVAL;
>> +
>> +	memset(vss->buffer + pos, 0, count);
>> +
>> +	return virtsnd_pcm_msg_send_locked(vss, true);
>> +}
>> +
>> +/* PCM substream operators map for playback. */
>> +const struct snd_pcm_ops virtsnd_pcm_playback_ops = {
>> +	.open = virtsnd_pcm_open,
>> +	.close = virtsnd_pcm_close,
>> +	.ioctl = snd_pcm_lib_ioctl,
>> +	.hw_params = virtsnd_pcm_hw_params,
>> +	.hw_free = virtsnd_pcm_hw_free,
>> +	.prepare = virtsnd_pcm_prepare,
>> +	.trigger = virtsnd_pcm_trigger,
>> +	.sync_stop = virtsnd_pcm_sync_stop,
>> +	.pointer = virtsnd_pcm_pointer,
>> +	.copy = virtsnd_pcm_pb_copy,
>> +	.fill_silence = virtsnd_pcm_pb_silence,
>> +};
>> +
>> +/* PCM substream operators map for capturing. */
>> +const struct snd_pcm_ops virtsnd_pcm_capture_ops = {
>>  	.open = virtsnd_pcm_open,
>>  	.close = virtsnd_pcm_close,
>>  	.ioctl = snd_pcm_lib_ioctl,
>> @@ -461,4 +532,5 @@ const struct snd_pcm_ops virtsnd_pcm_ops = {
>>  	.trigger = virtsnd_pcm_trigger,
>>  	.sync_stop = virtsnd_pcm_sync_stop,
>>  	.pointer = virtsnd_pcm_pointer,
>> +	.copy = virtsnd_pcm_cap_copy,
>>  };
>>
>> base-commit: 8a749fd1a8720d4619c91c8b6e7528c0a355c0aa
>> --
>> 2.41.0
>