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From 1ddf0b1b11aa8a90cef6706e935fc31c75c406ba Mon Sep 17 00:00:00 2001
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From: Andy Lutomirski <luto@amacapital.net>
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Date: Sun, 21 Dec 2014 08:57:46 -0800
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Subject: x86, vdso: Use asm volatile in __getcpu
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In Linux 3.18 and below, GCC hoists the lsl instructions in the
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pvclock code all the way to the beginning of __vdso_clock_gettime,
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slowing the non-paravirt case significantly. For unknown reasons,
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presumably related to the removal of a branch, the performance issue
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is gone as of
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e76b027e6408 x86,vdso: Use LSL unconditionally for vgetcpu
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but I don't trust GCC enough to expect the problem to stay fixed.
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There should be no correctness issue, because the __getcpu calls in
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__vdso_vlock_gettime were never necessary in the first place.
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Note to stable maintainers: In 3.18 and below, depending on
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configuration, gcc 4.9.2 generates code like this:
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9c3: 44 0f 03 e8 lsl %ax,%r13d
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9c7: 45 89 eb mov %r13d,%r11d
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9ca: 0f 03 d8 lsl %ax,%ebx
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This patch won't apply as is to any released kernel, but I'll send a
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trivial backported version if needed.
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Fixes: 51c19b4f5927 x86: vdso: pvclock gettime support
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Cc: stable@vger.kernel.org # 3.8+
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Cc: Marcelo Tosatti <mtosatti@redhat.com>
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Acked-by: Paolo Bonzini <pbonzini@redhat.com>
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Signed-off-by: Andy Lutomirski <luto@amacapital.net>
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(Backported to 3.17.8 by Josh Boyer <jwboyer@fedoraproject.org>)
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diff --git a/arch/x86/include/asm/vsyscall.h b/arch/x86/include/asm/vsyscall.h
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index 2a46ca720afc..3db1fa5fdceb 100644
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--- a/arch/x86/include/asm/vsyscall.h
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+++ b/arch/x86/include/asm/vsyscall.h
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@@ -34,7 +34,7 @@ static inline unsigned int __getcpu(void)
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native_read_tscp(&p);
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} else {
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/* Load per CPU data from GDT */
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- asm("lsl %1,%0" : "=r" (p) : "r" (__PER_CPU_SEG));
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+ asm volatile("lsl %1,%0" : "=r" (p) : "r" (__PER_CPU_SEG));
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}
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return p;
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