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Message-ID: <941293fc-de40-4143-9a5b-fa262736abee@csgroup.eu>
Date: Sat, 14 Jun 2025 08:42:49 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Takashi Iwai <tiwai@...e.de>
Cc: Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.com>,
linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
linux-sound@...r.kernel.org, Herve Codina <herve.codina@...tlin.com>
Subject: Re: [PATCH v2 2/4] ALSA: pcm: Convert SNDRV_PCM_IOCTL_SYNC_PTR to
user_access_begin/user_access_end()
Le 13/06/2025 à 18:41, Takashi Iwai a écrit :
> On Fri, 13 Jun 2025 17:37:09 +0200,
> Christophe Leroy wrote:
>>
>> 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>
>
> Hm, with this patch, I got a compile warning:
>
> sound/core/snd-pcm.o: warning: objtool: .altinstr_replacement+0x12: redundant UACCESS disable
Oops. My mistake.
Objtool doesn't check that on powerpc and I only checked build on x86
and arm64 by cross-compiling pcm_native.o. Should have done a complete
build.
user_read_access_end() has to be skipped when user_read_access_begin()
fails. Need to add a second label for that in the macros. Will send an
updated version of this patch. Have to send the entire series again
because change to patch 2 conflicts with patch 3.
Christophe
>
> It's with gcc-13.3.1.
>
>
> Takashi
>
>> ---
>> 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 20326aa377b0..fd69eea935b2 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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