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 e76b027e 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: 51c19b4f x86: vdso: pvclock gettime support Cc: stable@vger.kernel.org # 3.8+ Cc: Marcelo Tosatti <mtosatti@redhat.com> Acked-by: NPaolo Bonzini <pbonzini@redhat.com> Signed-off-by: NAndy Lutomirski <luto@amacapital.net>
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