Commit-ID: 1ddf0b1b11aa8a90cef6706e935fc31c75c406ba
Gitweb: http://git.kernel.org/tip/1ddf0b1b11aa8a90cef6706e935fc31c75c406ba
Author: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
AuthorDate: Sun, 21 Dec 2014 08:57:46 -0800
Committer: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
CommitDate: Tue, 23 Dec 2014 13:05:30 -0800
x86, vdso: Use asm volatile in __getcpu
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.
Fixes: 51c19b4f5927 x86: vdso: pvclock gettime support
Cc: stable@xxxxxxxxxxxxxxx # 3.8+
Cc: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
---
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 e7e9682..f556c48 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;
}
--
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