[RFC PATCH v2 0/1] ALSA: virtio: trace control message lifecycles

[Joonho Lee <mynameisjune111@gmail.com>]

Hi,

This adds a control-message lifecycle trace event to virtio-snd. A per-send
ID connects enqueue, response, cancellation and sender wait outcomes,
including a response arriving after a timeout.

Changes in v2:
- Group internal helper event arguments in struct virtsnd_ctl_event.
- Use designated initializers and named validity flags instead of positional
  booleans. Keep TRACE_EVENT and its record format unchanged.

The event definition, error handling and request lifetime are unchanged
from v1. The patch applies directly to the base below and has no dependency
on the experimental VZ capture changes.

Validation of v2:
- ARM64 W=1 builds, including KASAN/lockdep, BPF events and tracing-disabled
  configurations, passed without warnings or errors.
- Eight QEMU boots passed malformed-response, enqueue failure, asynchronous
  completion, interrupted wait, timeout and reset/recovery checks. Trace-on
  fixture runs recorded 280 events each without loss or KASAN/lockdep reports.
- A non-debug QEMU/HVF comparison completed 90,000 lifecycle cycles and three
  10-second synthetic captures at 4,096 frames. All functional checks passed;
  captures each read 480,000 frames with no XRUNs or read errors.

On GCC 14.2.0 ARM64, the event arguments are scalarized. The control object
text shrinks by 56 bytes and the send stack frame drops from 192 to 176 bytes.
With event tracing compiled out, the old and new object text is identical.
The existing 24-byte per-request metadata cost remains unchanged.

Host-load stability checks failed during the timing comparison, so those
measurements do not establish an isolated runtime overhead percentage.
These regressions use synthetic QEMU devices; native VZ steady-capture
performance and physical microphone acceptance remain unverified.

The malformed-length fixture writes a full response and changes only the
reported used length. The sender retains its existing behavior; this patch
records diagnostic fields without changing protocol validation.

Previous version:
https://lore.kernel.org/linux-sound/rfc-virtio-snd-control-trace-0.208f05ae8ea44e77@gmail.com/T/

Thanks,
Joonho

Joonho Lee (1):
  ALSA: virtio: trace control message lifecycles

 sound/virtio/Makefile               |   2 +
 sound/virtio/virtio_ctl_msg.c       | 178 +++++++++++++++++++++++++++++++++++-
 sound/virtio/virtio_ctl_msg_trace.h | 107 ++++++++++++++++++++++
 3 files changed, 286 insertions(+), 1 deletion(-)

base-commit: 2f2f7eeef8b0a1a9b9ca998231419873ccae8d4e