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| author | Andy Lutomirski <luto@amacapital.net> | 2014-12-21 08:57:46 -0800 |
|---|---|---|
| committer | Jiri Slaby <jslaby@suse.cz> | 2015-01-26 14:38:53 +0100 |
| commit | 779cf1d93e1194b041cd139b8e71e0baae4884a6 (patch) | |
| tree | e82050a269651377df67a952de2f1de218ff6060 /arch | |
| parent | 0c9312164d6e069707dd96a001849f4b853c546d (diff) | |
| download | lwn-779cf1d93e1194b041cd139b8e71e0baae4884a6.tar.gz lwn-779cf1d93e1194b041cd139b8e71e0baae4884a6.zip | |
x86, vdso: Use asm volatile in __getcpu
commit 1ddf0b1b11aa8a90cef6706e935fc31c75c406ba upstream.
In Linux 3.18 and below, GCC hoists the lsl instructions in the
pvclock code all the way to the beginning of __vdso_clock_gettime,
slowing the non-paravirt case significantly. For unknown reasons,
presumably related to the removal of a branch, the performance issue
is gone as of
e76b027e6408 x86,vdso: Use LSL unconditionally for vgetcpu
but I don't trust GCC enough to expect the problem to stay fixed.
There should be no correctness issue, because the __getcpu calls in
__vdso_vlock_gettime were never necessary in the first place.
Note to stable maintainers: In 3.18 and below, depending on
configuration, gcc 4.9.2 generates code like this:
9c3: 44 0f 03 e8 lsl %ax,%r13d
9c7: 45 89 eb mov %r13d,%r11d
9ca: 0f 03 d8 lsl %ax,%ebx
This patch won't apply as is to any released kernel, but I'll send a
trivial backported version if needed.
[
Backported by Andy Lutomirski. Should apply to all affected
versions. This fixes a functionality bug as well as a performance
bug: buggy kernels can infinite loop in __vdso_clock_gettime on
affected compilers. See, for exammple:
https://bugzilla.redhat.com/show_bug.cgi?id=1178975
]
Fixes: 51c19b4f5927 x86: vdso: pvclock gettime support
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Diffstat (limited to 'arch')
| -rw-r--r-- | arch/x86/include/asm/vsyscall.h | 2 |
1 files changed, 1 insertions, 1 deletions
diff --git a/arch/x86/include/asm/vsyscall.h b/arch/x86/include/asm/vsyscall.h index 2a46ca720afc..2874be9aef0a 100644 --- a/arch/x86/include/asm/vsyscall.h +++ b/arch/x86/include/asm/vsyscall.h @@ -34,7 +34,7 @@ static inline unsigned int __getcpu(void) native_read_tscp(&p); } else { /* Load per CPU data from GDT */ - asm("lsl %1,%0" : "=r" (p) : "r" (__PER_CPU_SEG)); + asm volatile ("lsl %1,%0" : "=r" (p) : "r" (__PER_CPU_SEG)); } return p; |