[RFC 1/2] x86, vdso: Use asm volatile in __getcpu

Andy Lutomirski
2014-12-23T00:39:56Z

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.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---
arch/x86/include/asm/vgtod.h | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/vgtod.h b/arch/x86/include/asm/vgtod.h
index e7e9682a33e9..f556c4843aa1 100644
--- a/arch/x86/include/asm/vgtod.h
+++ b/arch/x86/include/asm/vgtod.h
@@ -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;
}
--
2.1.0