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 a5cbdc1. 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>

README.md

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}

README.md

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'

virtio-snd.c

jserv

Refine git commit messages with explicit limitations.

cubic-dev-ai

2 issues found across 4 files

Prompt for AI agents (all issues)


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


<file name=".ci/test-sound.sh">

<violation number="1" location=".ci/test-sound.sh:47">
P1: Exit code is captured from `echo` instead of `test_sound`. With `set -e`, if `test_sound` fails the script exits immediately, making the error messages dead code. The exit status should be captured right after `test_sound`.</violation>
</file>

<file name="README.md">

<violation number="1" location="README.md:16">
P3: Grammatical issue: The sentence structure is broken. &quot;As a confirmed issue that... causing... will get stuck&quot; doesn&#39;t form a proper sentence. Consider rephrasing to: &quot;Due to a confirmed issue where `semu` cannot send/receive PCM frames in time, the ALSA stack may get stuck in XRUN state until you reboot `semu`.&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 · 2025-12-22T16:12:51Z

.ci/test-sound.sh

+test_sound
+echo "✓ sound test passed"
+
+ret="$?"


P1: Exit code is captured from echo instead of test_sound. With set -e, if test_sound fails the script exits immediately, making the error messages dead code. The exit status should be captured right after test_sound.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At .ci/test-sound.sh, line 47: <comment>Exit code is captured from `echo` instead of `test_sound`. With `set -e`, if `test_sound` fails the script exits immediately, making the error messages dead code. The exit status should be captured right after `test_sound`.</comment> <file context> @@ -0,0 +1,62 @@ +test_sound +echo "✓ sound test passed" + +ret="$?" + +MESSAGES=("OK!" \ </file context>

cubic-dev-ai · 2025-12-22T16:12:51Z

README.md

+        - As a confirmed issue that `semu` cannot send/receive PCM frames in time, causing
+          the ALSA stack will get stuck in XRUN state until you reboot `semu`.


P3: Grammatical issue: The sentence structure is broken. "As a confirmed issue that... causing... will get stuck" doesn't form a proper sentence. Consider rephrasing to: "Due to a confirmed issue where semu cannot send/receive PCM frames in time, the ALSA stack may get stuck in XRUN state until you reboot semu."

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At README.md, line 16: <comment>Grammatical issue: The sentence structure is broken. "As a confirmed issue that... causing... will get stuck" doesn't form a proper sentence. Consider rephrasing to: "Due to a confirmed issue where `semu` cannot send/receive PCM frames in time, the ALSA stack may get stuck in XRUN state until you reboot `semu`."</comment> <file context> @@ -12,12 +12,13 @@ A minimalist RISC-V system emulator capable of running Linux the kernel and corr - to adjust the buffer size to more than four times of period size, or - 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 will get stuck in XRUN state until you reboot `semu`. + - For playback, you can try to adjust the buffer size to more than four times of period size. </file context>

Suggested change

- As a confirmed issue that `semu` cannot send/receive PCM frames in time, causing

the ALSA stack will get stuck in XRUN state until you reboot `semu`.

- Due to a confirmed issue where `semu` cannot send/receive PCM frames in time,

the ALSA stack may get stuck in XRUN state until you reboot `semu`.

cubic-dev-ai

2 issues found across 4 files

Prompt for AI agents (all 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:16">
P3: This sentence has grammatical issues that make it hard to parse. Consider restructuring to: &quot;Due to a confirmed issue, `semu` cannot send/receive PCM frames in time, causing the ALSA stack to get stuck in XRUN state until you reboot `semu`.&quot;</violation>
</file>

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

<violation number="1" location="virtio-snd.c:1230">
P1: PortAudio callbacks must be non-blocking. `__virtio_snd_rx_frame_enqueue` uses `pthread_cond_wait` which can block indefinitely when called from the real-time audio callback `virtio_snd_rx_stream_cb`. This can cause audio glitches, buffer underruns, or deadlocks. Consider using a lock-free ring buffer or non-blocking queue for the audio callback path.</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 · 2025-12-22T16:16:58Z

README.md

-          to adjust the buffer size to more than four times of period size, or
-          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


P3: This sentence has grammatical issues that make it hard to parse. Consider restructuring to: "Due to a confirmed issue, semu cannot send/receive PCM frames in time, causing the ALSA stack to get stuck in XRUN state until you reboot semu."

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At README.md, line 16: <comment>This sentence has grammatical issues that make it hard to parse. Consider restructuring to: "Due to a confirmed issue, `semu` cannot send/receive PCM frames in time, causing the ALSA stack to get stuck in XRUN state until you reboot `semu`."</comment> <file context> @@ -12,12 +12,13 @@ A minimalist RISC-V system emulator capable of running Linux the kernel and corr - to adjust the buffer size to more than four times of period size, or - 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 will get stuck in XRUN state until you reboot `semu`. + - For playback, you can try to adjust the buffer size to more than four times of period size. </file context>

Suggested change

- As a confirmed issue that `semu` cannot send/receive PCM frames in time, causing

- Due to a confirmed issue, `semu` cannot send/receive PCM frames in time, causing

cubic-dev-ai · 2025-12-22T16:16:58Z

virtio-snd.c

+    virtio_snd_prop_t *props = &vsnd_props[stream_id];
+
+    pthread_mutex_lock(&props->lock.lock);
+    while (props->lock.buf_ev_notity > 0)


P1: PortAudio callbacks must be non-blocking. __virtio_snd_rx_frame_enqueue uses pthread_cond_wait which can block indefinitely when called from the real-time audio callback virtio_snd_rx_stream_cb. This can cause audio glitches, buffer underruns, or deadlocks. Consider using a lock-free ring buffer or non-blocking queue for the audio callback path.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At virtio-snd.c, line 1230: <comment>PortAudio callbacks must be non-blocking. `__virtio_snd_rx_frame_enqueue` uses `pthread_cond_wait` which can block indefinitely when called from the real-time audio callback `virtio_snd_rx_stream_cb`. This can cause audio glitches, buffer underruns, or deadlocks. Consider using a lock-free ring buffer or non-blocking queue for the audio callback path.</comment> <file context> @@ -952,6 +1220,37 @@ static void __virtio_snd_frame_enqueue(void *payload, + virtio_snd_prop_t *props = &vsnd_props[stream_id]; + + pthread_mutex_lock(&props->lock.lock); + while (props->lock.buf_ev_notity > 0) + pthread_cond_wait(&props->lock.writable, &props->lock.lock); + </file context>

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Implement VirtIO sound device capture

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

		- As a confirmed issue that `semu` cannot send/receive PCM frames in time, causing
		the ALSA stack will get stuck in XRUN state until you reboot `semu`.

	- As a confirmed issue that `semu` cannot send/receive PCM frames in time, causing
	- Due to a confirmed issue, `semu` cannot send/receive PCM frames in time, causing

Implement VirtIO sound device capture #115

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Implement VirtIO sound device capture #115

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