Implement VirtIO sound device capture #115

Cuda-Chen · 2025-11-17T06:48:30Z

Summary by cubic

Implements VirtIO sound capture (RX) with PortAudio input, RX virtqueue handling, and circular buffering so input audio can be delivered to the guest. Also exposes separate input/output streams with accurate device info.

New Features
- Added RX virtqueue handlers (normal/flush) with default flush stream_id = 1.
- Opened PortAudio input stream and RX callback to capture frames and enqueue to the driver.
- Implemented circular intermediate buffer (buf_sz, buf_idx) and RX enqueue/dequeue helpers.
- Exposed two streams (output + input), mono S16, with per-stream jack/pcm/chmap info.
- Added RX worker thread, mutex/cond vars, and VSND_QUEUE_RX notifications.
- Applied direction-aware flush in pcm_release and set RX response length/status.
- Added CI sound tests for playback and capture and wired them into GitHub Actions.

^{Written for commit 41fe790. Summary will update automatically on new commits.}

jserv

Rebase to avoid unnecessary merge commits.

jserv · 2025-12-17T03:56:30Z

virtio-snd.c

+static void __virtio_snd_rx_frame_enqueue(void *payload,
+                                          uint32_t n,
+                                          uint32_t stream_id)


__virtio_snd_rx_frame_enqueue() never increments buf_ev_notity or signals the condition variable, but __virtio_snd_rx_frame_dequeue() waits on it:

// In __virtio_snd_rx_frame_dequeue: while (props->lock.buf_ev_notity < 1) pthread_cond_wait(&props->lock.readable, ...); // BLOCKS FOREVER

The TX path correctly does:

props->lock.buf_ev_notity++; pthread_cond_signal(&props->lock.readable);

Fix: Add after list_push() in __virtio_snd_rx_frame_enqueue:

props->lock.buf_ev_notity++; pthread_cond_signal(&props->lock.readable);

The buf_ev_notity will be increased and then signals the update of CV after consuming all the virtq elements when calling virtio_snd_rx_desc_normal_handler():

// In VSND_GEN_RX_QUEUE_HANDLER macro virtio_snd_prop_t *props = &vsnd_props[stream_id]; props->lock.buf_ev_notity--; pthread_cond_signal(&props->lock.writable);

The virtio_snd_tx_desc_normal_handler() also has the same design.

The reason is that the driver should populate a buffer size of PCM frames (except the end of stream) as mentioned by VirtIO standard:

5.14.6.8.2.2 Driver Requirements: Input Stream

The driver SHOULD populate the rx queue with period_bytes sized empty buffers before starting the stream.

The driver MUST NOT place device-readable buffers into the rx queue.

jserv · 2025-12-17T03:57:20Z

virtio-snd.c

+static void __virtio_snd_rx_frame_dequeue(void *out,
+                                          uint32_t n,
+                                          uint32_t stream_id)


__virtio_snd_rx_frame_dequeue() does list_del(&node->q) but never free(node):

if (node->pos >= node->len) list_del(&node->q); // Missing: free(node);

virtio-snd.c

jserv · 2025-12-17T03:59:22Z

Debug Printf Pollution
Multiple fprintf(stderr, ...) debug statements throughout the RX path should be removed before merge.

Makefile

virtio-snd.c

jserv · 2025-12-17T06:51:03Z

Can we perform loopback tests with virtio sound device?

Cuda-Chen · 2025-12-17T07:09:51Z

Can we perform loopback tests with virtio sound device?

You mean aloop or a device loopback testing command like arecord -C | aplay -?

jserv · 2025-12-17T07:11:38Z

You mean aloop of a device loopback testing command like arecord -C | aplay -?

The headless verification is crucial for CI/CD integration.

.ci/test-sound.sh

Implement VirtIO sound device capture, with adding notice that the capture usually doesn't work for the reason of 'semu' emulation part.

cubic-dev-ai

3 issues found across 4 files

Prompt for AI agents (all 3 issues)


Check if these issues are valid — if so, understand the root cause of each and fix them.


<file name="virtio-snd.c">

<violation number="1" location="virtio-snd.c:373">
P2: The global `rx_ev_start` has a data race - it&#39;s read in the RX thread (in `__virtio_snd_rx_frame_dequeue`) while being written from the control path without synchronization. Consider using atomic operations or protecting access with `props-&gt;lock.lock` mutex.</violation>
</file>

<file name="README.md">

<violation number="1" location="README.md:17">
P3: Grammar error: &quot;until `semu` being rebooted&quot; should be &quot;until `semu` is rebooted&quot;.</violation>

<violation number="2" location="README.md:21">
P3: Grammar error: subject-verb agreement. &quot;ALSA usually get stuck&quot; should be &quot;ALSA usually gets stuck&quot;.</violation>
</file>

_{Reply to cubic to teach it or ask questions. Re-run a review with @cubic-dev-ai review this PR}

cubic-dev-ai · 2025-12-19T10:02:39Z

virtio-snd.c

+static int rx_ev_notify;
+
+// FIXME: set these variables into each capture stream;s structure
+static int rx_ev_start;


P2: The global rx_ev_start has a data race - it's read in the RX thread (in __virtio_snd_rx_frame_dequeue) while being written from the control path without synchronization. Consider using atomic operations or protecting access with props->lock.lock mutex.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At virtio-snd.c, line 373: <comment>The global `rx_ev_start` has a data race - it's read in the RX thread (in `__virtio_snd_rx_frame_dequeue`) while being written from the control path without synchronization. Consider using atomic operations or protecting access with `props->lock.lock` mutex.</comment> <file context> @@ -316,9 +362,15 @@ static virtio_snd_prop_t vsnd_props[VSND_DEV_CNT_MAX] = { +static int rx_ev_notify; + +// FIXME: set these variables into each capture stream;s structure +static int rx_ev_start; /* vsnd virtq callback type */ </file context>

cubic-dev-ai · 2025-12-19T10:02:39Z

README.md

+        - For playback, you can try to adjust the buffer size to more than four times of period size.
            - For instance, the following buffer/period size settings on `aplay` has been tested
              with broken and stutter effects yet complete with no any errors: `aplay --buffer-size=32768 --period-size=4096 /usr/share/sounds/alsa/Front_Center.wav`.
+        - For capture, ALSA usually get stuck in XRUN state, so you may need to try multiple times. 


P3: Grammar error: subject-verb agreement. "ALSA usually get stuck" should be "ALSA usually gets stuck".

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At README.md, line 21: <comment>Grammar error: subject-verb agreement. "ALSA usually get stuck" should be "ALSA usually gets stuck".</comment> <file context> @@ -12,12 +12,13 @@ A minimalist RISC-V system emulator capable of running Linux the kernel and corr + - For playback, you can try to adjust the buffer size to more than four times of period size. - For instance, the following buffer/period size settings on `aplay` has been tested with broken and stutter effects yet complete with no any errors: `aplay --buffer-size=32768 --period-size=4096 /usr/share/sounds/alsa/Front_Center.wav`. + - For capture, ALSA usually get stuck in XRUN state, so you may need to try multiple times. ## Prerequisites </file context>

Suggested change

- For capture, ALSA usually get stuck in XRUN state, so you may need to try multiple times.

- For capture, ALSA usually gets stuck in XRUN state, so you may need to try multiple times.

cubic-dev-ai · 2025-12-19T10:02:39Z

README.md

-          the program cannot write the PCM frames into guest OS ALSA stack.
+    - virtio-snd uses [PortAudio](https://github.com/PortAudio/portaudio) for sound playback and capture on the host with one limitation:
+        - As a confirmed issue that `semu` cannot send/receive PCM frames in time, causing
+          the ALSA stack stuck in XRUN state until `semu` being rebooted.


P3: Grammar error: "until semu being rebooted" should be "until semu is rebooted".

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At README.md, line 17: <comment>Grammar error: "until `semu` being rebooted" should be "until `semu` is rebooted".</comment> <file context> @@ -12,12 +12,13 @@ A minimalist RISC-V system emulator capable of running Linux the kernel and corr - the program cannot write the PCM frames into guest OS ALSA stack. + - virtio-snd uses [PortAudio](https://github.com/PortAudio/portaudio) for sound playback and capture on the host with one limitation: + - As a confirmed issue that `semu` cannot send/receive PCM frames in time, causing + the ALSA stack stuck in XRUN state until `semu` being rebooted. + - For playback, you can try to adjust the buffer size to more than four times of period size. - For instance, the following buffer/period size settings on `aplay` has been tested </file context>

Suggested change

the ALSA stack stuck in XRUN state until `semu` being rebooted.

the ALSA stack to get stuck in XRUN state until `semu` is rebooted.

cubic-dev-ai · 2025-12-19T10:04:00Z

@cubic-dev-ai Review

@jserv I have started the AI code review. It will take a few minutes to complete.

cubic-dev-ai

5 issues found across 4 files

Prompt for AI agents (all 5 issues)


Check if these issues are valid — if so, understand the root cause of each and fix them.


<file name="README.md">

<violation number="1" location="README.md:17">
P3: Grammar issue: &#39;stuck&#39; should be &#39;to get stuck&#39; and &#39;being rebooted&#39; should be &#39;is rebooted&#39;.</violation>

<violation number="2" location="README.md:21">
P3: Subject-verb agreement error: &quot;ALSA usually get stuck&quot; should be &quot;ALSA usually gets stuck&quot;.</violation>
</file>

<file name=".github/workflows/main.yml">

<violation number="1" location=".github/workflows/main.yml:72">
P1: The sound test runs unconditionally for all matrix variants, including when `matrix.dependency == &#39;none&#39;` where no sound library is installed. This will likely cause the test to fail. Consider adding a condition similar to the install step.</violation>
</file>

<file name="virtio-snd.c">

<violation number="1" location="virtio-snd.c:857">
P1: Potential double-free: `props-&gt;intermediate` is freed in error path but not set to NULL. If `virtio_snd_read_pcm_release` is called later (state is already PREPARE), it will free the same pointer again.</violation>

<violation number="2" location="virtio-snd.c:1225">
P1: Missing NULL checks after malloc. If memory allocation fails, dereferencing `node` or `node-&gt;addr` will cause a crash.</violation>
</file>

_{Reply to cubic to teach it or ask questions. Re-run a review with @cubic-dev-ai review this PR}

cubic-dev-ai · 2025-12-19T10:12:16Z

README.md

+        - For playback, you can try to adjust the buffer size to more than four times of period size.
            - For instance, the following buffer/period size settings on `aplay` has been tested
              with broken and stutter effects yet complete with no any errors: `aplay --buffer-size=32768 --period-size=4096 /usr/share/sounds/alsa/Front_Center.wav`.
+        - For capture, ALSA usually get stuck in XRUN state, so you may need to try multiple times. 


P3: Subject-verb agreement error: "ALSA usually get stuck" should be "ALSA usually gets stuck".

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At README.md, line 21: <comment>Subject-verb agreement error: "ALSA usually get stuck" should be "ALSA usually gets stuck".</comment> <file context> @@ -12,12 +12,13 @@ A minimalist RISC-V system emulator capable of running Linux the kernel and corr + - For playback, you can try to adjust the buffer size to more than four times of period size. - For instance, the following buffer/period size settings on `aplay` has been tested with broken and stutter effects yet complete with no any errors: `aplay --buffer-size=32768 --period-size=4096 /usr/share/sounds/alsa/Front_Center.wav`. + - For capture, ALSA usually get stuck in XRUN state, so you may need to try multiple times. ## Prerequisites </file context>

Suggested change

- For capture, ALSA usually get stuck in XRUN state, so you may need to try multiple times.

- For capture, ALSA usually gets stuck in XRUN state, so you may need to try multiple times.

cubic-dev-ai · 2025-12-19T10:12:16Z

README.md

-          the program cannot write the PCM frames into guest OS ALSA stack.
+    - virtio-snd uses [PortAudio](https://github.com/PortAudio/portaudio) for sound playback and capture on the host with one limitation:
+        - As a confirmed issue that `semu` cannot send/receive PCM frames in time, causing
+          the ALSA stack stuck in XRUN state until `semu` being rebooted.


P3: Grammar issue: 'stuck' should be 'to get stuck' and 'being rebooted' should be 'is rebooted'.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At README.md, line 17: <comment>Grammar issue: 'stuck' should be 'to get stuck' and 'being rebooted' should be 'is rebooted'.</comment> <file context> @@ -12,12 +12,13 @@ A minimalist RISC-V system emulator capable of running Linux the kernel and corr - the program cannot write the PCM frames into guest OS ALSA stack. + - virtio-snd uses [PortAudio](https://github.com/PortAudio/portaudio) for sound playback and capture on the host with one limitation: + - As a confirmed issue that `semu` cannot send/receive PCM frames in time, causing + the ALSA stack stuck in XRUN state until `semu` being rebooted. + - For playback, you can try to adjust the buffer size to more than four times of period size. - For instance, the following buffer/period size settings on `aplay` has been tested </file context>

Suggested change

the ALSA stack stuck in XRUN state until `semu` being rebooted.

the ALSA stack to get stuck in XRUN state until `semu` is rebooted.

cubic-dev-ai · 2025-12-19T10:12:16Z

.github/workflows/main.yml

      run: sudo .ci/test-netdev.sh
      shell: bash
      timeout-minutes: 10
+    - name: sound test


P1: The sound test runs unconditionally for all matrix variants, including when matrix.dependency == 'none' where no sound library is installed. This will likely cause the test to fail. Consider adding a condition similar to the install step.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At .github/workflows/main.yml, line 72: <comment>The sound test runs unconditionally for all matrix variants, including when `matrix.dependency == 'none'` where no sound library is installed. This will likely cause the test to fail. Consider adding a condition similar to the install step.</comment> <file context> @@ -69,6 +69,10 @@ jobs: run: sudo .ci/test-netdev.sh shell: bash timeout-minutes: 10 + - name: sound test + run: .ci/test-sound.sh + shell: bash </file context>

Suggested change

- name: sound test

- name: sound test

if: matrix.dependency != 'none'

cubic-dev-ai · 2025-12-19T10:12:16Z

virtio-snd.c

+        pthread_cond_wait(&props->lock.writable, &props->lock.lock);
+
+    /* Add a PCM frame to queue */
+    vsnd_buf_queue_node_t *node = malloc(sizeof(*node));


P1: Missing NULL checks after malloc. If memory allocation fails, dereferencing node or node->addr will cause a crash.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At virtio-snd.c, line 1225: <comment>Missing NULL checks after malloc. If memory allocation fails, dereferencing `node` or `node->addr` will cause a crash.</comment> <file context> @@ -952,6 +1211,29 @@ static void __virtio_snd_frame_enqueue(void *payload, + pthread_cond_wait(&props->lock.writable, &props->lock.lock); + + /* Add a PCM frame to queue */ + vsnd_buf_queue_node_t *node = malloc(sizeof(*node)); + node->addr = malloc(sizeof(*node->addr) * n); + memcpy(node->addr, payload, n); </file context>

cubic-dev-ai · 2025-12-19T10:12:16Z

virtio-snd.c

+    return;
+
+pa_err:
+    free(props->intermediate);


P1: Potential double-free: props->intermediate is freed in error path but not set to NULL. If virtio_snd_read_pcm_release is called later (state is already PREPARE), it will free the same pointer again.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At virtio-snd.c, line 857: <comment>Potential double-free: `props->intermediate` is freed in error path but not set to NULL. If `virtio_snd_read_pcm_release` is called later (state is already PREPARE), it will free the same pointer again.</comment> <file context> @@ -650,27 +806,57 @@ static void virtio_snd_read_pcm_prepare(const virtio_snd_pcm_hdr_t *query, + return; + +pa_err: + free(props->intermediate); + fprintf(stderr, "Cannot create PortAudio\n"); + printf("PortAudio error: %s\n", Pa_GetErrorText(err)); </file context>

Suggested change

free(props->intermediate);

free(props->intermediate);

props->intermediate = NULL;

jserv requested changes Nov 17, 2025

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Cuda-Chen force-pushed the add-virtio-snd-rx branch 2 times, most recently from 54a45b5 to 05b22f9 Compare November 17, 2025 10:31

Cuda-Chen force-pushed the add-virtio-snd-rx branch from 2db638a to 781ef0a Compare December 17, 2025 03:40

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Cuda-Chen force-pushed the add-virtio-snd-rx branch 2 times, most recently from be341b2 to 1a14913 Compare December 17, 2025 06:16

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Cuda-Chen force-pushed the add-virtio-snd-rx branch 3 times, most recently from fd4f538 to a2348ec Compare December 18, 2025 11:02

Cuda-Chen marked this pull request as ready for review December 18, 2025 11:46

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Implement VirtIO sound device capture

41fe790

Implement VirtIO sound device capture, with adding notice that the capture usually doesn't work for the reason of 'semu' emulation part.

Cuda-Chen force-pushed the add-virtio-snd-rx branch from 30d1ccd to 41fe790 Compare December 18, 2025 13:04

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	- For capture, ALSA usually get stuck in XRUN state, so you may need to try multiple times.
	- For capture, ALSA usually gets stuck in XRUN state, so you may need to try multiple times.

	the ALSA stack stuck in XRUN state until `semu` being rebooted.
	the ALSA stack to get stuck in XRUN state until `semu` is rebooted.

	- name: sound test
	- name: sound test
	if: matrix.dependency != 'none'

	free(props->intermediate);
	free(props->intermediate);
	props->intermediate = NULL;

Implement VirtIO sound device capture #115

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Implement VirtIO sound device capture #115

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