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

[Andy Lutomirski <luto@xxxxxxxxxxxxxx>]

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@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 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


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