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x86, vdso: Use asm volatile in __getcpu

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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

e76b027e64 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.

Fixes: 51c19b4f59 x86: vdso: pvclock gettime support
Cc: stable@vger.kernel.org # 3.8+
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
This commit is contained in:
Andy Lutomirski 2014-12-21 08:57:46 -08:00
parent 394f56fe48
commit 1ddf0b1b11
1 changed files with 4 additions and 2 deletions
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6
arch/x86/include/asm/vgtod.h
View file

@ -80,9 +80,11 @@ static inline unsigned int __getcpu(void)
/*
* Load per CPU data from GDT. LSL is faster than RDTSCP and
* works on all CPUs.
* works on all CPUs. This is volatile so that it orders
* correctly wrt barrier() and to keep gcc from cleverly
* hoisting it out of the calling function.
*/
asm("lsl %1,%0" : "=r" (p) : "r" (__PER_CPU_SEG));
asm volatile ("lsl %1,%0" : "=r" (p) : "r" (__PER_CPU_SEG));
return p;
}