summaryrefslogtreecommitdiffstats
path: root/arch/x86
diff options
context:
space:
mode:
authorAndy Lutomirski <luto@amacapital.net>2014-12-21 08:57:46 -0800
committerAndy Lutomirski <luto@amacapital.net>2014-12-23 13:05:30 -0800
commit1ddf0b1b11aa8a90cef6706e935fc31c75c406ba (patch)
tree78e79b96126a6f54c5f4afa37d4f11f07aaab6d8 /arch/x86
parent394f56fe480140877304d342dec46d50dc823d46 (diff)
downloadlinux-1ddf0b1b11aa8a90cef6706e935fc31c75c406ba.tar.gz
linux-1ddf0b1b11aa8a90cef6706e935fc31c75c406ba.tar.bz2
linux-1ddf0b1b11aa8a90cef6706e935fc31c75c406ba.zip
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>
Diffstat (limited to 'arch/x86')
-rw-r--r--arch/x86/include/asm/vgtod.h6
1 files 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;
}