From 44259b1abfaa8bb819d25d41d71e8e33e25dd36a Mon Sep 17 00:00:00 2001 From: Andy Lutomirski Date: Mon, 23 May 2011 09:31:28 -0400 Subject: x86-64: Move vread_tsc into a new file with sensible options vread_tsc is short and hot, and it's userspace code so the usual reasons to enable -pg and turn off sibling calls don't apply. (OK, turning off sibling calls has no effect. But it might someday...) As an added benefit, tsc.c is profilable now. Signed-off-by: Andy Lutomirski Cc: Andi Kleen Cc: Linus Torvalds Cc: "David S. Miller" Cc: Eric Dumazet Cc: Peter Zijlstra Cc: Borislav Petkov Link: http://lkml.kernel.org/r/%3C99c6d7f5efa3ccb65b4ac6eb443e1ab7bad47d7b.1306156808.git.luto%40mit.edu%3E Signed-off-by: Thomas Gleixner --- arch/x86/kernel/vread_tsc_64.c | 36 ++++++++++++++++++++++++++++++++++++ 1 file changed, 36 insertions(+) create mode 100644 arch/x86/kernel/vread_tsc_64.c (limited to 'arch/x86/kernel/vread_tsc_64.c') diff --git a/arch/x86/kernel/vread_tsc_64.c b/arch/x86/kernel/vread_tsc_64.c new file mode 100644 index 000000000000..a81aa9e9894c --- /dev/null +++ b/arch/x86/kernel/vread_tsc_64.c @@ -0,0 +1,36 @@ +/* This code runs in userspace. */ + +#define DISABLE_BRANCH_PROFILING +#include + +notrace cycle_t __vsyscall_fn vread_tsc(void) +{ + cycle_t ret; + u64 last; + + /* + * Empirically, a fence (of type that depends on the CPU) + * before rdtsc is enough to ensure that rdtsc is ordered + * with respect to loads. The various CPU manuals are unclear + * as to whether rdtsc can be reordered with later loads, + * but no one has ever seen it happen. + */ + rdtsc_barrier(); + ret = (cycle_t)vget_cycles(); + + last = VVAR(vsyscall_gtod_data).clock.cycle_last; + + if (likely(ret >= last)) + return ret; + + /* + * GCC likes to generate cmov here, but this branch is extremely + * predictable (it's just a funciton of time and the likely is + * very likely) and there's a data dependence, so force GCC + * to generate a branch instead. I don't barrier() because + * we don't actually need a barrier, and if this function + * ever gets inlined it will generate worse code. + */ + asm volatile (""); + return last; +} -- cgit v1.2.3-70-g09d2