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Message-ID: <79b581119343a12e417d7fe7b296aa50c84900cc.1749724478.git.christophe.leroy@csgroup.eu>
Date: Thu, 12 Jun 2025 12:51:04 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Jaroslav Kysela <perex@...ex.cz>,
Takashi Iwai <tiwai@...e.com>
Cc: Christophe Leroy <christophe.leroy@...roup.eu>,
linux-kernel@...r.kernel.org,
linuxppc-dev@...ts.ozlabs.org,
linux-sound@...r.kernel.org,
Herve Codina <herve.codina@...tlin.com>,
Mark Brown <broonie@...nel.org>
Subject: [RFC PATCH 2/3] ALSA: pcm: Convert SNDRV_PCM_IOCTL_SYNC_PTR to user_access_begin/user_access_end()
With user access protection (Called SMAP on x86 or KUAP on powerpc)
each and every call to get_user() or put_user() performs heavy
operations to unlock and lock kernel access to userspace.
SNDRV_PCM_IOCTL_SYNC_PTR is a hot path which is called really often
and needs to run as fast as possible.
To improve performance, perform user accesses by blocks using
user_access_begin/user_access_end() and unsafe_get_user()/
unsafe_put_user().
Before the patch the 9 calls to put_user() at the end of
snd_pcm_ioctl_sync_ptr_compat() imply the following set of
instructions about 9 times (access_ok - enable user - write - disable
user):
0.00 : c057f858: 3d 20 7f ff lis r9,32767
0.29 : c057f85c: 39 5e 00 14 addi r10,r30,20
0.77 : c057f860: 61 29 ff fc ori r9,r9,65532
0.32 : c057f864: 7c 0a 48 40 cmplw r10,r9
0.36 : c057f868: 41 a1 fb 58 bgt c057f3c0 <snd_pcm_ioctl+0xbb0>
0.30 : c057f86c: 3d 20 dc 00 lis r9,-9216
1.95 : c057f870: 7d 3a c3 a6 mtspr 794,r9
0.33 : c057f874: 92 8a 00 00 stw r20,0(r10)
0.27 : c057f878: 3d 20 de 00 lis r9,-8704
0.28 : c057f87c: 7d 3a c3 a6 mtspr 794,r9
...
A perf profile shows that in total the 9 put_user() represent 36% of
the time spent in snd_pcm_ioctl() and about 80 instructions.
With this patch everything is done in 13 instructions and represent
only 15% of the time spent in snd_pcm_ioctl():
0.57 : c057f5dc: 3d 20 dc 00 lis r9,-9216
0.98 : c057f5e0: 7d 3a c3 a6 mtspr 794,r9
0.16 : c057f5e4: 92 7f 00 04 stw r19,4(r31)
0.63 : c057f5e8: 93 df 00 0c stw r30,12(r31)
0.16 : c057f5ec: 93 9f 00 10 stw r28,16(r31)
4.95 : c057f5f0: 92 9f 00 14 stw r20,20(r31)
0.19 : c057f5f4: 92 5f 00 18 stw r18,24(r31)
0.49 : c057f5f8: 92 bf 00 1c stw r21,28(r31)
0.27 : c057f5fc: 93 7f 00 20 stw r27,32(r31)
5.88 : c057f600: 93 36 00 00 stw r25,0(r22)
0.11 : c057f604: 93 17 00 00 stw r24,0(r23)
0.00 : c057f608: 3d 20 de 00 lis r9,-8704
0.79 : c057f60c: 7d 3a c3 a6 mtspr 794,r9
Note that here the access_ok() in user_write_access_begin() is skipped
because the exact same verification has already been performed at the
beginning of the fonction with the call to user_read_access_begin().
Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
---
sound/core/pcm_native.c | 42 +++++++++++++++++++++++++----------------
1 file changed, 26 insertions(+), 16 deletions(-)
diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
index 2ea31df0c46d..554352f546c9 100644
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -3053,30 +3053,40 @@ static inline int snd_pcm_hwsync(struct snd_pcm_substream *substream)
}
#define snd_pcm_sync_ptr_get_user(__f, __c, __ptr) ({ \
- int err = 0; \
+ __label__ failed; \
+ int err = -EFAULT; \
typeof(*__ptr) __user *__src = (__ptr); \
\
- if (get_user(__f, &src->flags) || \
- get_user(__c.appl_ptr, &__src->c.control.appl_ptr) || \
- get_user(__c.avail_min, &__src->c.control.avail_min)) \
- err = -EFAULT; \
+ if (!user_read_access_begin(__src, sizeof(*__src))) \
+ goto failed; \
+ unsafe_get_user(__f, &__src->flags, failed); \
+ unsafe_get_user(__c.appl_ptr, &__src->c.control.appl_ptr, failed); \
+ unsafe_get_user(__c.avail_min, &__src->c.control.avail_min, failed); \
+ err = 0; \
+failed: \
+ user_read_access_end(); \
err; \
})
#define snd_pcm_sync_ptr_put_user(__s, __c, __ptr) ({ \
- int err = 0; \
+ __label__ failed; \
+ int err = -EFAULT; \
typeof(*__ptr) __user *__src = (__ptr); \
\
- if (put_user(__s.state, &__src->s.status.state) || \
- put_user(__s.hw_ptr, &__src->s.status.hw_ptr) || \
- put_user(__s.tstamp.tv_sec, &__src->s.status.tstamp_sec) || \
- put_user(__s.tstamp.tv_nsec, &__src->s.status.tstamp_nsec) || \
- put_user(__s.suspended_state, &__src->s.status.suspended_state) ||\
- put_user(__s.audio_tstamp.tv_sec, &__src->s.status.audio_tstamp_sec) ||\
- put_user(__s.audio_tstamp.tv_nsec, &__src->s.status.audio_tstamp_nsec) ||\
- put_user(__c.appl_ptr, &__src->c.control.appl_ptr) || \
- put_user(__c.avail_min, &__src->c.control.avail_min)) \
- err = -EFAULT; \
+ if (!user_write_access_begin(__src, sizeof(*__src))) \
+ goto failed; \
+ unsafe_put_user(__s.state, &__src->s.status.state, failed); \
+ unsafe_put_user(__s.hw_ptr, &__src->s.status.hw_ptr, failed); \
+ unsafe_put_user(__s.tstamp.tv_sec, &__src->s.status.tstamp_sec, failed);\
+ unsafe_put_user(__s.tstamp.tv_nsec, &__src->s.status.tstamp_nsec, failed); \
+ unsafe_put_user(__s.suspended_state, &__src->s.status.suspended_state, failed); \
+ unsafe_put_user(__s.audio_tstamp.tv_sec, &__src->s.status.audio_tstamp_sec, failed); \
+ unsafe_put_user(__s.audio_tstamp.tv_nsec, &__src->s.status.audio_tstamp_nsec, failed); \
+ unsafe_put_user(__c.appl_ptr, &__src->c.control.appl_ptr, failed); \
+ unsafe_put_user(__c.avail_min, &__src->c.control.avail_min, failed); \
+ err = 0; \
+failed: \
+ user_write_access_end(); \
err; \
})
--
2.47.0
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