[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] [RFC 1/2] 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. 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 _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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