Implement VirtIO sound device capture by Cuda-Chen · Pull Request #115 · sysprog21/semu

Cuda-Chen · 2025-11-17T06:48:30Z

Summary by cubic

Implements VirtIO sound capture (RX) with PortAudio input and full RX virtqueue support so mic audio reaches the guest. Adds wake-fd IRQ signaling and keeps TX/RX fully split with separate paths and threads.

New Features
- RX virtqueue handler and TX/RX flush handlers; default flush streams: TX=0, RX=1.
- Separate PortAudio callbacks; RX opens the default input device and uses the guest’s period_bytes.
- TX/RX buffer queues with enqueue/dequeue helpers; RX dequeues only while the stream is running.
- Dedicated TX and RX worker threads with separate mutex/cond vars and VSND_QUEUE_TX/RX notifications; added wake_fd to signal IRQs and wake the host loop.
- Two streams (output + input), mono S16; README adds an ALSA XRUN note for capture.

^{Written for commit 352b5e2. Summary will update on new commits.}

jserv · 2025-12-17T03:57:20Z

+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);

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.

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

      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'

jserv

VSND_GEN_RX_QUEUE_HANDLER is 116 lines of macro that's 95% identical to VSND_GEN_TX_QUEUE_HANDLER. This is not "good taste."

jserv · 2025-12-24T04:35:23Z

commit message:

Due to 'semu' emulation part thus 'semu' cannot send or receive PCM frames in time, the capture feature usually
doesn't work.

You're adding 400+ lines of code that "usually doesn't work"? This should be a draft/experimental branch, not a merge candidate. If it doesn't work, why are we adding CI tests that codify broken behavior?

jserv · 2025-12-24T04:36:11Z

Missing Lock on TX Notify Path:

  case VSND_QUEUE_TX:
      tx_ev_notify++;  // NO LOCK!
      pthread_cond_signal(&virtio_snd_tx_cond);
      break;

  // virtio-snd.c:1455 - RX path (new code)
  case VSND_QUEUE_RX:
      pthread_mutex_lock(&virtio_snd_rx_mutex);  // HAS LOCK
      rx_ev_notify++;
      pthread_cond_signal(&virtio_snd_rx_cond);
      pthread_mutex_unlock(&virtio_snd_rx_mutex);
      break;

Inconsistent synchronization patterns between TX and RX. TX is already racy - RX is correct but highlights the TX bug.

cubic-dev-ai

3 issues found across 1 file (changes from recent commits).

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="virtio-snd.c">

<violation number="1" location="virtio-snd.c:1132">
P1: Using `pthread_mutex_trylock` silently drops audio frames when the lock is unavailable. This will cause audio data loss and artifacts in the capture stream. Consider either using blocking `pthread_mutex_lock` with a timeout, or implementing a lock-free ring buffer for the real-time audio callback.</violation>

<violation number="2" location="virtio-snd.c:1133">
P2: Debug `fprintf` in the real-time audio callback path should be removed. Calling `fprintf` from a PortAudio callback can cause audio glitches as it may block.</violation>

<violation number="3" location="virtio-snd.c:1152">
P2: Debug `fprintf` in the real-time audio callback path should be removed.</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 · 2025-12-24T09:36:08Z

+        pthread_cond_signal(&props->lock.readable);
+
+    rx_frame_enque_finally:
+        fprintf(stderr, "enque end\n");


P2: Debug fprintf in the real-time audio callback path should be removed.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At virtio-snd.c, line 1152: <comment>Debug `fprintf` in the real-time audio callback path should be removed.</comment> <file context> @@ -1119,29 +1126,32 @@ static void __virtio_snd_rx_frame_enqueue(const void *payload, -rx_frame_enque_finally: - pthread_mutex_unlock(&props->lock.lock); + rx_frame_enque_finally: + fprintf(stderr, "enque end\n"); + pthread_mutex_unlock(&props->lock.lock); + } </file context>

cubic-dev-ai · 2025-12-24T09:36:08Z

+    while (props->lock.buf_ev_notity > 0)
+        pthread_cond_wait(&props->lock.writable, &props->lock.lock);*/
+    if (pthread_mutex_trylock(&props->lock.lock) == 0) {
+        fprintf(stderr, "enque start \n");


P2: Debug fprintf in the real-time audio callback path should be removed. Calling fprintf from a PortAudio callback can cause audio glitches as it may block.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At virtio-snd.c, line 1133: <comment>Debug `fprintf` in the real-time audio callback path should be removed. Calling `fprintf` from a PortAudio callback can cause audio glitches as it may block.</comment> <file context> @@ -1119,29 +1126,32 @@ static void __virtio_snd_rx_frame_enqueue(const void *payload, - list_push(&node->q, &props->buf_queue_head); + pthread_cond_wait(&props->lock.writable, &props->lock.lock);*/ + if (pthread_mutex_trylock(&props->lock.lock) == 0) { + fprintf(stderr, "enque start \n"); + /* Add a PCM frame to queue */ + vsnd_buf_queue_node_t *node = malloc(sizeof(*node)); </file context>

cubic-dev-ai · 2025-12-24T09:36:08Z

+    /*pthread_mutex_lock(&props->lock.lock);
+    while (props->lock.buf_ev_notity > 0)
+        pthread_cond_wait(&props->lock.writable, &props->lock.lock);*/
+    if (pthread_mutex_trylock(&props->lock.lock) == 0) {


P1: Using pthread_mutex_trylock silently drops audio frames when the lock is unavailable. This will cause audio data loss and artifacts in the capture stream. Consider either using blocking pthread_mutex_lock with a timeout, or implementing a lock-free ring buffer for the real-time audio callback.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At virtio-snd.c, line 1132: <comment>Using `pthread_mutex_trylock` silently drops audio frames when the lock is unavailable. This will cause audio data loss and artifacts in the capture stream. Consider either using blocking `pthread_mutex_lock` with a timeout, or implementing a lock-free ring buffer for the real-time audio callback.</comment> <file context> @@ -1119,29 +1126,32 @@ static void __virtio_snd_rx_frame_enqueue(const void *payload, - node->pos = 0; - list_push(&node->q, &props->buf_queue_head); + pthread_cond_wait(&props->lock.writable, &props->lock.lock);*/ + if (pthread_mutex_trylock(&props->lock.lock) == 0) { + fprintf(stderr, "enque start \n"); + /* Add a PCM frame to queue */ </file context>

jserv · 2026-03-09T10:56:34Z

 #include "virtio.h"

-#define VSND_DEV_CNT_MAX 1
+#define VSND_DEV_CNT_MAX 2


Provide comments to explain.

This PR is going to add capture device. As each VirtIO sound device can have only one direction when initialization (VIRTIO_SND_D_OUTPUT or VIRTIO_SND_D_INPUT), I set VSND_DEV_CNT_MAX to 2 (one for playback, and the other for capture).

jserv · 2026-03-09T10:58:30Z

+        err = Pa_OpenStream(&props->pa_stream, &params, NULL /* no output */,
+                            rate, cnfa_period_frames, paClipOff,
+                            virtio_snd_rx_stream_cb, &props->v);
+        if (err != paNoError)
+            goto pa_err;


Can you provide early diagnosis when PortAudio fails?

cubic-dev-ai · 2026-05-31T03:07:37Z

You're iterating quickly on this pull request. To help protect your rate limits, cubic has paused automatic reviews on new pushes for now—when you're ready for another review, comment @cubic-dev-ai review.

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[WIP] checking missing pcm0c

a5b8265

Cuda-Chen force-pushed the add-virtio-snd-rx branch from 429d9d2 to a5b8265 Compare May 19, 2026 15:20

[WIP] Remove reduntent #if guard

352b5e2

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

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