# Guest-side build of the patched sound.ko (transcript of the live session;
# the guest disk was reverted afterwards, so this is the captured record):
#   (after applying fix.diff to /usr/src/sys/dev/sound/pcm/channel.c)
#   cd /usr/src/sys/dev/sound/sound && make cleandir; make obj; make -j4 depend; make -j4 all
# ...
# --- sound.ko ---
# cc -Wl,--build-id=sha1 -nostdlib -Wl,--hash-style=sysv -r -Wl,-d -o sound.ko ac97_if.o \
#   channel_if.o feeder_if.o mixer_if.o feeder.o feeder_rate.o feeder_volume.o feeder_chain.o \
#   feeder_eq.o feeder_format.o feeder_matrix.o feeder_mixer.o mpu_if.o mpufoi_if.o synth_if.o \
#   ac97.o ac97_patch.o buffer.o channel.o clone.o dsp.o mixer.o sndstat.o sound.o unit.o \
#   vchan.o midi.o mpu401.o sequencer.o
# MD5 (/usr/obj/usr/src/sys/dev/sound/sound/sound.ko) = 0d076339abf53061007aa1deeecedfd7
# nm: 000000000000c420 T chn_trigger
# install -o root -g wheel -m 555 sound.ko /boot/kernel/sound.ko
# MD5 (/boot/kernel/sound.ko) = 0d076339abf53061007aa1deeecedfd7
# source check: grep SAFE(d, c, channels.pcm.busy) channel.c
#   2201: CHN_INSERT_HEAD_SAFE(d, c, channels.pcm.busy);
#   2218: CHN_REMOVE_SAFE(d, c, channels.pcm.busy);