56 lines
2.0 KiB
Diff
56 lines
2.0 KiB
Diff
From 1ddf0b1b11aa8a90cef6706e935fc31c75c406ba Mon Sep 17 00:00:00 2001
|
|
From: Andy Lutomirski <luto@amacapital.net>
|
|
Date: Sun, 21 Dec 2014 08:57:46 -0800
|
|
Subject: 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@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>
|
|
|
|
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;
|
|
}
|
|
--
|
|
cgit v0.10.2
|
|
|