mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
synced 2026-05-14 02:59:19 -04:00
a9b49bf8ad
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@csgroup.eu>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/eccd047f2dfeb550129a1d60035e2233c4401d0c.1749883041.git.christophe.leroy@csgroup.eu
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.