[PATCH 0/2] ALSA: virtio-snd: fix dsnoop capture with staged buffers

[PATCH 0/2] ALSA: virtio-snd: fix dsnoop capture with staged buffers

[Joonho Lee]

Hi,

This series fixes dsnoop capture on top of the staged-capture virtio-snd
changes. dsnoop configures its slave with stop_threshold == boundary and
can keep capture running without advancing the slave application pointer as
an ordinary reader would. The staged-capture accounting currently exhausts
its bounded unread capacity in this configuration.

The first patch marks stop_threshold accesses used by the driver's policy
sampling. READ_ONCE/WRITE_ONCE provide single-access annotations; the
virtio-snd state transitions remain protected by the substream lock.

The second patch caches the prepared boundary and samples the stop policy
under that lock. A completion uses one sampled policy for validation,
copying and queue refill. Free-running capture keeps requests queued and
retains the existing frame, payload and pointer-window checks. Returning to
bounded capture restores unread accounting from the monotonic counters. If
the outstanding data already fills the ring, the policy helper returns
-EPIPE without changing the cached mode or counters. The detecting RX,
pointer or ACK path then reports XRUN through its existing error path.
This does not guarantee a synchronous error from the sw_params ioctl.

The ACK accounting and subsequent queue refill each sample the policy under
the appropriate locks. Prepare and teardown reset the cached state. The
existing ACK-race and priority changes are prerequisites to this series and
are excluded from these two diffs.

Testing:

* ARM64 QEMU synthetic dsnoop capture: 12 files and 2,160,000 frames
  matched the reference sequence from each client's observed starting
  position. All 48 corruption, loss, duplication and reordering controls
  were detected.
* Default KCSAN: 13/13 cases and 619 policy transitions passed without
  race or lockdep reports. Forced RX/ACK policy changes and four isolated
  policy-helper branches passed under KASAN and KCSAN.
* macOS VZ with MOTU UltraLite-mk5 at 48 kHz: comparisons used one or two
  vCPUs and idle or CPU-saturated guests. The baseline failed all 32
  shared-client captures. The candidate passed 48 captures and eight
  recovery diagnostics, including mixed-rate sharing, client departure,
  reopening and recovery after an intentionally delayed reader.
  Loaded shared captures ran for 70 seconds. All eight accepted VM runs
  shut down on the first request.

The physical tests checked frame counts and nonzero input. Exact sample
continuity was checked with the QEMU synthetic source. VZ exposed mono;
these tests do not establish physical input-channel routing or multichannel
support. Aggressive KCSAN still produced dsnoop client-threshold XRUNs with
a 160 ms buffer. Further isolation supports a deadline sensitivity, but
does not exclude every kernel timing contribution. Separate generic KCSAN
reports remain recorded in the test evidence.

This series depends on the staged-capture prerequisite stack below. The
tested local base is 8d0d4c1a6c4bcb81ae9f52d1c0d979fafb2e22f2, consisting
of nine prerequisite patches above sound.git commit
5618f6837ef3314e2c581d4c05ec13b7ed975cb4.

Prerequisites, in application order:

1-5. Lifetime and error recovery v3:
https://lore.kernel.org/all/cover.1791881787.git.mynameisjune111@gmail.com/
6. Generic teardown/control wait v2:
https://lore.kernel.org/all/rfc-v2-generic-control-wait-0.2eddd58a796e497f@gmail.com/
7. VZ staged capture v2:
https://lore.kernel.org/all/rfc-v2-vz-capture-compat-1.b9ab4c1540dc4691@gmail.com/
8-9. ACK accounting and high-priority period work:
https://lore.kernel.org/all/rfc-virtio-snd-capture-followups-1.f2737c95c12e45b4@gmail.com/
https://lore.kernel.org/all/rfc-virtio-snd-capture-followups-2.f2737c95c12e45b4@gmail.com/

This fixes the staged-capture accounting introduced by that prerequisite
stack. The reproductions do not establish the same failure in an unmodified
upstream kernel.

The policy uses the existing stop_threshold == boundary convention.
Sampling it under the driver lock avoids introducing a PCM stream lock into
RX completion. A completion overlapping a policy update may finish with the
previous policy; its accounting and refill use that same sample. Rejecting
a transition whose bounded accounting cannot be restored preserves
explicit overflow detection and lets the caller recover the stream.

Patches:
  ALSA: pcm: mark stop-threshold accesses for capture policy sampling
  ALSA: virtio-snd: fix dsnoop capture with staged buffers

Thanks,
Joonho

[PATCH 1/2] ALSA: pcm: mark stop-threshold accesses for capture policy sampling

[Joonho Lee]

The staged virtio-snd capture path needs to sample stop_threshold from
completion context to distinguish free-running capture from bounded
capture. Software parameters can update this field while capture runs.

Mark the runtime stop-threshold assignments with WRITE_ONCE and the proc
status read with READ_ONCE. This pairs the assignments with the driver's
READ_ONCE policy sampling in the following patch.

These annotations do not provide a transaction for the entire sw_params
update. The driver serializes its cached policy and accounting with its
own substream lock.

Assisted-by: LLM
Signed-off-by: Joonho Lee <mynameisjune111@gmail.com>

---
diff --git a/sound/core/oss/pcm_oss.c b/sound/core/oss/pcm_oss.c
--- a/sound/core/oss/pcm_oss.c
+++ b/sound/core/oss/pcm_oss.c
@@ -2958,7 +2958,7 @@
 	       runtime->oss.mmap_bytes);
 #endif
 	/* In mmap mode we never stop */
-	runtime->stop_threshold = runtime->boundary;
+	WRITE_ONCE(runtime->stop_threshold, runtime->boundary);
 
 	return 0;
 }
diff --git a/sound/core/pcm.c b/sound/core/pcm.c
--- a/sound/core/pcm.c
+++ b/sound/core/pcm.c
@@ -422,7 +422,8 @@
 	snd_iprintf(buffer, "period_step: %u\n", runtime->period_step);
 	snd_iprintf(buffer, "avail_min: %lu\n", runtime->control->avail_min);
 	snd_iprintf(buffer, "start_threshold: %lu\n", runtime->start_threshold);
-	snd_iprintf(buffer, "stop_threshold: %lu\n", runtime->stop_threshold);
+	snd_iprintf(buffer, "stop_threshold: %lu\n",
+		    READ_ONCE(runtime->stop_threshold));
 	snd_iprintf(buffer, "silence_threshold: %lu\n", runtime->silence_threshold);
 	snd_iprintf(buffer, "silence_size: %lu\n", runtime->silence_size);
 	snd_iprintf(buffer, "boundary: %lu\n", runtime->boundary);
diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -871,7 +871,7 @@
 	runtime->period_step = 1;
 	runtime->control->avail_min = runtime->period_size;
 	runtime->start_threshold = 1;
-	runtime->stop_threshold = runtime->buffer_size;
+	WRITE_ONCE(runtime->stop_threshold, runtime->buffer_size);
 	runtime->silence_threshold = 0;
 	runtime->silence_size = 0;
 	runtime->boundary = runtime->buffer_size;
@@ -1062,7 +1062,8 @@
 		runtime->period_step = params->period_step;
 		runtime->control->avail_min = params->avail_min;
 		runtime->start_threshold = params->start_threshold;
-		runtime->stop_threshold = params->stop_threshold;
+		/* Drivers may sample the stop policy from completion context. */
+		WRITE_ONCE(runtime->stop_threshold, params->stop_threshold);
 		runtime->silence_threshold = params->silence_threshold;
 		runtime->silence_size = params->silence_size;
 		params->boundary = runtime->boundary;

[PATCH 2/2] ALSA: virtio-snd: fix dsnoop capture with staged buffers

[Joonho Lee]

dsnoop runs its hardware slave with stop_threshold == boundary and can
consume the mmap capture ring without advancing the hardware application
pointer. Staged capture currently treats every stream as a bounded
consumer: capture_ready grows until the completion capacity check fails.

With a 48 kHz mono slave, 960-frame periods and a 7680-frame buffer, the
baseline dsnoop reproduction stops after 6720 frames with -EPIPE and a
ring_capacity rejection. Direct capture continues to work.

Cache the prepared boundary and recognize the free-running policy under
the substream lock. Keep RX requests supplied in that mode, while retaining
payload, frame alignment, counter and pointer-window checks. Sample the
policy once per completion so validation, copying and refill agree.
Refresh it separately in the ACK accounting and refill critical sections.

On a return to bounded capture, reconstruct unread occupancy from the
copied and acknowledged totals. If outstanding data fills the ring, the
policy helper returns -EPIPE and preserves the cached mode and counters.
The detecting RX, pointer or ACK path reports XRUN through its existing
error path; this does not make sw_params fail synchronously.
Reset cached policy state during prepare and teardown.

In MOTU/VZ comparisons with one and two vCPUs, the baseline failed all
32 shared-client captures. The candidate passed 48 capture cases and eight
recovery diagnostics, including 70-second CPU-saturated shared captures.
Synthetic QEMU dsnoop data matched 2160000 reference frames across 12
files. Forced policy transitions passed KASAN/KCSAN checks. Aggressive
KCSAN still triggered dsnoop client-threshold XRUNs with a 160 ms buffer.
The physical tests checked frame counts and nonzero mono input; exact
sample continuity was checked with the synthetic source.

Assisted-by: LLM
Signed-off-by: Joonho Lee <mynameisjune111@gmail.com>

---
diff --git a/sound/virtio/virtio_pcm.h b/sound/virtio/virtio_pcm.h
--- a/sound/virtio/virtio_pcm.h
+++ b/sound/virtio/virtio_pcm.h
@@ -47,7 +47,9 @@
  *        substream operators, including @stopped.
  * @buffer_bytes: Current buffer size in bytes.
  * @hw_ptr: Substream hardware pointer value in bytes [0 ... buffer_bytes).
- * @capture_ready: Captured bytes copied but not yet acknowledged by ALSA.
+ * @capture_ready: Managed-mode bytes copied but not yet acknowledged by ALSA;
+ *                 always zero in free-running mode.
+ * @capture_boundary: ALSA boundary captured for this prepared epoch.
  * @capture_copied_total: Monotonic bytes copied into the capture ring.
  * @capture_acked_total: Monotonic bytes acknowledged by the capture client.
  * @capture_pointer_seen_total: Last capture position sampled by ALSA.
@@ -58,6 +60,7 @@
  * @capture_staging: Capture uses request-owned payload buffers.
  * @capture_started: Non-empty capture data was received since prepare.
  * @capture_empty: Consecutive empty startup completions.
+ * @capture_free_running: Cached staged-capture policy, protected by @lock.
  * @xfer_enabled: Data transfer state (0 - off, 1 - on).
  * @xfer_xrun: Data underflow/overflow state (0 - no xrun, 1 - xrun).
  * @stopped: True if the substream is stopped and must be released on the device
@@ -88,6 +91,7 @@
 	size_t buffer_bytes;
 	size_t hw_ptr;
 	size_t capture_ready;
+	snd_pcm_uframes_t capture_boundary;
 	u64 capture_copied_total;
 	u64 capture_acked_total;
 	u64 capture_pointer_seen_total;
@@ -97,6 +101,7 @@
 	bool runtime_active;
 	bool capture_staging;
 	bool capture_started;
+	bool capture_free_running;
 	unsigned int capture_empty;
 	bool xfer_enabled;
 	bool xfer_xrun;
@@ -128,23 +133,29 @@
 
 /*
  * ALSA may observe a subperiod tail before its period callback. Keep actual
- * unread occupancy separate from pointer observations and notification state.
+ * unread occupancy separate from pointer observations and notification state;
+ * free-running streams account occupancy from monotonic totals instead.
  */
 static inline bool
 virtsnd_pcm_capture_counters_valid(const struct virtio_pcm_substream *vss)
 {
-	return vss->buffer_bytes &&
-	       vss->capture_copied_total >= vss->capture_acked_total &&
-	       vss->capture_copied_total - vss->capture_acked_total ==
-			(u64)vss->capture_ready &&
-	       vss->capture_ready < vss->buffer_bytes &&
-	       vss->capture_pointer_seen_total <= vss->capture_copied_total &&
-	       vss->hw_ptr == vss->capture_copied_total % vss->buffer_bytes &&
-	       (!vss->capture_callback_snapshot_active ||
-		(vss->capture_callback_snapshot >=
-			vss->capture_pointer_seen_total &&
-		 vss->capture_callback_snapshot <=
-			vss->capture_copied_total));
+	u64 outstanding;
+
+	if (!vss->buffer_bytes ||
+	    vss->capture_copied_total < vss->capture_acked_total ||
+	    vss->capture_pointer_seen_total > vss->capture_copied_total ||
+	    vss->hw_ptr != vss->capture_copied_total % vss->buffer_bytes ||
+	    (vss->capture_callback_snapshot_active &&
+	     (vss->capture_callback_snapshot < vss->capture_pointer_seen_total ||
+	      vss->capture_callback_snapshot > vss->capture_copied_total)))
+		return false;
+
+	if (vss->capture_free_running)
+		return !vss->capture_ready;
+
+	outstanding = vss->capture_copied_total - vss->capture_acked_total;
+	return outstanding == (u64)vss->capture_ready &&
+	       vss->capture_ready < vss->buffer_bytes;
 }
 
 /**
@@ -207,6 +218,9 @@
 
 int virtsnd_pcm_capture_send(struct virtio_pcm_substream *vss);
 
+int virtsnd_pcm_capture_refresh_mode_locked(struct virtio_pcm_substream *vss,
+					    bool *free_running);
+
 void virtsnd_pcm_msg_free(struct virtio_pcm_substream *vss);
 
 void virtsnd_pcm_msg_reset(struct virtio_pcm_substream *vss);
diff --git a/sound/virtio/virtio_pcm_msg.c b/sound/virtio/virtio_pcm_msg.c
--- a/sound/virtio/virtio_pcm_msg.c
+++ b/sound/virtio/virtio_pcm_msg.c
@@ -3,6 +3,7 @@
  * virtio-snd: Virtio sound device
  * Copyright (C) 2021 OpenSynergy GmbH
  */
+#include <linux/compiler.h>
 #include <linux/limits.h>
 #include <linux/mm.h>
 #include <linux/overflow.h>
@@ -384,6 +385,8 @@
 	vss->capture_notified_periods = 0;
 	vss->capture_callback_snapshot = 0;
 	vss->capture_callback_snapshot_active = false;
+	vss->capture_boundary = 0;
+	vss->capture_free_running = false;
 	vss->capture_empty = 0;
 	vss->capture_started = false;
 }
@@ -508,24 +511,54 @@
 	return 0;
 }
 
-/**
- * virtsnd_pcm_capture_send() - Queue staged capture requests.
- * @vss: VirtIO PCM substream.
- *
- * Queue full-period requests while the outstanding requests fit the remaining
- * ALSA ring capacity. The final request may exceed that capacity by less than
- * one period; completion validates the returned payload before copying it.
- *
- * Context: Expects the RX queue and substream spinlocks to be held by caller.
- * Return: 0 on success, -errno on failure.
- */
-int virtsnd_pcm_capture_send(struct virtio_pcm_substream *vss)
+/* Caller holds vss->lock. */
+int virtsnd_pcm_capture_refresh_mode_locked(struct virtio_pcm_substream *vss,
+					    bool *free_running)
+{
+	struct snd_pcm_runtime *runtime;
+	snd_pcm_uframes_t stop_threshold;
+	u64 outstanding;
+	bool new_free_running;
+
+	if (!vss->capture_staging || !vss->runtime_active || !vss->substream)
+		return -EBADFD;
+
+	runtime = vss->substream->runtime;
+	if (!runtime || !vss->buffer_bytes || !vss->capture_boundary)
+		return -EBADFD;
+
+	/* Validate the currently committed policy before observing a transition. */
+	if (!virtsnd_pcm_capture_counters_valid(vss))
+		return -EIO;
+
+	stop_threshold = READ_ONCE(runtime->stop_threshold);
+	new_free_running = stop_threshold == vss->capture_boundary;
+	if (new_free_running != vss->capture_free_running) {
+		outstanding = vss->capture_copied_total -
+			      vss->capture_acked_total;
+		if (new_free_running) {
+			vss->capture_ready = 0;
+		} else {
+			if (outstanding >= vss->buffer_bytes)
+				return -EPIPE;
+			vss->capture_ready = outstanding;
+		}
+		vss->capture_free_running = new_free_running;
+	}
+
+	*free_running = new_free_running;
+	return 0;
+}
+
+/* Caller holds the RX queue and vss locks and has sampled the policy. */
+static int virtsnd_pcm_capture_enqueue(struct virtio_pcm_substream *vss,
+				       bool free_running)
 {
 	struct virtio_snd_queue *queue = virtsnd_pcm_queue(vss);
 	struct virtqueue *vqueue = queue->vqueue;
 	unsigned long period_bytes;
 	size_t free_bytes;
-	size_t request_limit;
+	size_t request_limit = vss->nmsgs;
 	unsigned int i;
 	bool added = false;
 
@@ -535,16 +568,17 @@
 	period_bytes = snd_pcm_lib_period_bytes(vss->substream);
 	if (!period_bytes || !vss->buffer_bytes ||
 	    period_bytes >= vss->buffer_bytes ||
+	    free_running != vss->capture_free_running ||
 	    !virtsnd_pcm_capture_counters_valid(vss) ||
 	    vss->msg_count > vss->nmsgs)
 		return -EIO;
 
-	free_bytes = vss->buffer_bytes - vss->capture_ready;
-	request_limit = free_bytes / period_bytes +
-			!!(free_bytes % period_bytes);
-	request_limit = min_t(size_t, request_limit, vss->nmsgs);
-	if (vss->msg_count > request_limit)
-		return -EIO;
+	if (!free_running) {
+		free_bytes = vss->buffer_bytes - vss->capture_ready;
+		request_limit = free_bytes / period_bytes +
+				!!(free_bytes % period_bytes);
+		request_limit = min_t(size_t, request_limit, vss->nmsgs);
+	}
 
 	for (i = 0; i < vss->nmsgs && vss->msg_count < request_limit; ++i) {
 		struct virtio_pcm_msg *msg = vss->msgs[i];
@@ -585,6 +619,25 @@
 	return 0;
 }
 
+/**
+ * virtsnd_pcm_capture_send() - Refresh policy and queue staged requests.
+ * @vss: VirtIO PCM substream.
+ *
+ * Context: Expects the RX queue and substream spinlocks to be held by caller.
+ * Return: 0 on success, -errno on failure.
+ */
+int virtsnd_pcm_capture_send(struct virtio_pcm_substream *vss)
+{
+	bool free_running;
+	int rc;
+
+	rc = virtsnd_pcm_capture_refresh_mode_locked(vss, &free_running);
+	if (rc)
+		return rc;
+
+	return virtsnd_pcm_capture_enqueue(vss, free_running);
+}
+
 static int virtsnd_pcm_capture_status(const struct virtio_pcm_msg *msg,
 				      size_t written_bytes,
 				      size_t frame_bytes,
@@ -638,6 +691,7 @@
 	size_t audio_bytes = 0;
 	size_t next_ready = 0;
 	u64 next_copied_total = 0;
+	bool free_running = false;
 	bool got_audio = false;
 
 	guard(spinlock)(&vss->lock);
@@ -651,6 +705,9 @@
 		/* A full ring is modulo-ambiguous, so fail closed before copying. */
 		if (!runtime || !runtime->dma_area)
 			invalid_reason = "runtime";
+		else if (virtsnd_pcm_capture_refresh_mode_locked(vss,
+								 &free_running))
+			invalid_reason = "policy";
 		else if (!virtsnd_pcm_capture_counters_valid(vss))
 			invalid_reason = "counters";
 		else if (vss->hw_ptr !=
@@ -660,18 +717,28 @@
 						    frame_bytes, &audio_bytes,
 						    &latency_bytes))
 			invalid_reason = "status";
-		else if (audio_bytes >=
-			 vss->buffer_bytes - vss->capture_ready)
-			invalid_reason = "ring_capacity";
-		else if (check_add_overflow(vss->capture_ready, audio_bytes,
-					    &next_ready))
-			invalid_reason = "ready_overflow";
-		else if (next_ready >= vss->buffer_bytes)
-			invalid_reason = "ring_full";
 		else if (check_add_overflow(vss->capture_copied_total,
 					    (u64)audio_bytes,
 					    &next_copied_total))
 			invalid_reason = "total_overflow";
+		else if (free_running && audio_bytes >= vss->buffer_bytes)
+			invalid_reason = "payload_exceeds_ring";
+		else if (!free_running &&
+			 audio_bytes >= vss->buffer_bytes - vss->capture_ready)
+			invalid_reason = "ring_capacity";
+		else if (!free_running &&
+			 check_add_overflow(vss->capture_ready, audio_bytes,
+					    &next_ready))
+			invalid_reason = "ready_overflow";
+		else if (!free_running && next_ready >= vss->buffer_bytes)
+			invalid_reason = "ring_full";
+		else if (free_running &&
+			 (next_copied_total < vss->capture_pointer_seen_total ||
+			  next_copied_total - vss->capture_pointer_seen_total >=
+			  vss->buffer_bytes))
+			invalid_reason = "pointer_window";
+		else if (free_running)
+			next_ready = 0;
 
 		if (invalid_reason) {
 			virtsnd_pcm_capture_xrun_locked(vss);
@@ -713,7 +780,8 @@
 
 	msg->length = 0;
 	if (vss->runtime_active && vss->xfer_enabled) {
-		if (!vss->xfer_xrun && virtsnd_pcm_capture_send(vss)) {
+		if (!vss->xfer_xrun &&
+		    virtsnd_pcm_capture_enqueue(vss, free_running)) {
 			dev_warn_ratelimited(&vss->snd->vdev->dev,
 					     "SID %u: failed to refill capture queue\n",
 					     vss->sid);
diff --git a/sound/virtio/virtio_pcm_ops.c b/sound/virtio/virtio_pcm_ops.c
--- a/sound/virtio/virtio_pcm_ops.c
+++ b/sound/virtio/virtio_pcm_ops.c
@@ -543,6 +543,8 @@
 			vss->capture_notified_periods = 0;
 			vss->capture_callback_snapshot = 0;
 			vss->capture_callback_snapshot_active = false;
+			vss->capture_boundary = 0;
+			vss->capture_free_running = false;
 		}
 	}
 
@@ -565,6 +567,7 @@
 	struct virtio_pcm_substream *vss = snd_pcm_substream_chip(substream);
 	struct virtio_device *vdev = vss->snd->vdev;
 	bool pause_restart = virtsnd_pcm_pause_restart_enabled(vss);
+	bool free_running;
 	int rc;
 	bool stopped;
 
@@ -622,6 +625,15 @@
 	vss->suspended = false;
 	vss->msg_count = 0;
 	virtsnd_pcm_msg_reset(vss);
+	if (vss->capture_staging) {
+		scoped_guard(spinlock_irqsave, &vss->lock) {
+			vss->capture_boundary = substream->runtime->boundary;
+			rc = virtsnd_pcm_capture_refresh_mode_locked(vss,
+								     &free_running);
+		}
+		if (rc)
+			return rc;
+	}
 
 	memset(&vss->pcm_indirect, 0, sizeof(vss->pcm_indirect));
 	vss->pcm_indirect.sw_buffer_size =
@@ -1132,12 +1144,18 @@
 	struct virtio_pcm_substream *vss = snd_pcm_substream_chip(substream);
 	size_t hw_ptr = 0;
 	u64 total;
+	bool free_running;
 	bool xrun;
 
 	if (vss->capture_staging) {
 		scoped_guard(spinlock_irqsave, &vss->lock) {
-			xrun = vss->xfer_xrun ||
-			       !virtsnd_pcm_capture_counters_valid(vss);
+			xrun = vss->xfer_xrun;
+			if (!xrun &&
+			    virtsnd_pcm_capture_refresh_mode_locked(vss,
+								    &free_running))
+				xrun = true;
+			if (!xrun && !virtsnd_pcm_capture_counters_valid(vss))
+				xrun = true;
 			if (!xrun) {
 				total = vss->capture_callback_snapshot_active ?
 					vss->capture_callback_snapshot :
@@ -1244,14 +1262,18 @@
 		snd_pcm_uframes_t appl_ptr;
 		snd_pcm_sframes_t diff;
 		size_t bytes;
+		u64 outstanding;
 		u64 next_acked_total;
+		bool free_running;
 
 		if (!vss->runtime_active || !runtime || !runtime->control ||
 		    !runtime->dma_area)
 			return -EBADFD;
 		if (vss->xfer_xrun)
 			return -EPIPE;
-		if (!virtsnd_pcm_capture_counters_valid(vss)) {
+		rc = virtsnd_pcm_capture_refresh_mode_locked(vss,
+							     &free_running);
+		if (rc || !virtsnd_pcm_capture_counters_valid(vss)) {
 			vss->xfer_xrun = true;
 			virtsnd_pcm_schedule_period_elapsed(vss);
 			return -EPIPE;
@@ -1270,7 +1292,11 @@
 
 		/* ACK includes any subperiod tail already copied to the ring. */
 		bytes = frames_to_bytes(runtime, diff);
-		if (bytes > vss->capture_ready ||
+		outstanding = vss->capture_copied_total -
+			      vss->capture_acked_total;
+		if ((!free_running && bytes > vss->capture_ready) ||
+		    (free_running &&
+		     (bytes > vss->buffer_bytes || bytes > outstanding)) ||
 		    check_add_overflow(vss->capture_acked_total, (u64)bytes,
 				       &next_acked_total) ||
 		    next_acked_total > vss->capture_copied_total) {
@@ -1280,7 +1306,8 @@
 		}
 
 		vss->capture_acked_total = next_acked_total;
-		vss->capture_ready -= bytes;
+		if (!free_running)
+			vss->capture_ready -= bytes;
 		vss->pcm_indirect.appl_ptr = appl_ptr;
 	}