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Subject[PATCH v5 5/8] x86-64: Move vread_tsc into a new file with sensible options
Date
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 <luto@mit.edu>
---
arch/x86/include/asm/tsc.h | 4 ++++
arch/x86/kernel/Makefile | 8 +++++---
arch/x86/kernel/tsc.c | 34 ----------------------------------
arch/x86/kernel/vread_tsc_64.c | 36 ++++++++++++++++++++++++++++++++++++
4 files changed, 45 insertions(+), 37 deletions(-)
create mode 100644 arch/x86/kernel/vread_tsc_64.c
diff --git a/arch/x86/include/asm/tsc.h b/arch/x86/include/asm/tsc.h
index 83e2efd..9db5583 100644
--- a/arch/x86/include/asm/tsc.h
+++ b/arch/x86/include/asm/tsc.h
@@ -51,6 +51,10 @@ extern int unsynchronized_tsc(void);
extern int check_tsc_unstable(void);
extern unsigned long native_calibrate_tsc(void);

+#ifdef CONFIG_X86_64
+extern cycles_t vread_tsc(void);
+#endif
+
/*
* Boot-time check whether the TSCs are synchronized across
* all CPUs/cores:
diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index 7338ef2..a24521b 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -8,7 +8,6 @@ CPPFLAGS_vmlinux.lds += -U$(UTS_MACHINE)

ifdef CONFIG_FUNCTION_TRACER
# Do not profile debug and lowlevel utilities
-CFLAGS_REMOVE_tsc.o = -pg
CFLAGS_REMOVE_rtc.o = -pg
CFLAGS_REMOVE_paravirt-spinlocks.o = -pg
CFLAGS_REMOVE_pvclock.o = -pg
@@ -24,13 +23,16 @@ endif
nostackp := $(call cc-option, -fno-stack-protector)
CFLAGS_vsyscall_64.o := $(PROFILING) -g0 $(nostackp)
CFLAGS_hpet.o := $(nostackp)
-CFLAGS_tsc.o := $(nostackp)
+CFLAGS_vread_tsc_64.o := $(nostackp)
CFLAGS_paravirt.o := $(nostackp)
GCOV_PROFILE_vsyscall_64.o := n
GCOV_PROFILE_hpet.o := n
GCOV_PROFILE_tsc.o := n
GCOV_PROFILE_paravirt.o := n

+# vread_tsc_64 is hot and should be fully optimized:
+CFLAGS_REMOVE_vread_tsc_64.o = -pg -fno-optimize-sibling-calls
+
obj-y := process_$(BITS).o signal.o entry_$(BITS).o
obj-y += traps.o irq.o irq_$(BITS).o dumpstack_$(BITS).o
obj-y += time.o ioport.o ldt.o dumpstack.o
@@ -39,7 +41,7 @@ obj-$(CONFIG_IRQ_WORK) += irq_work.o
obj-$(CONFIG_X86_32) += probe_roms_32.o
obj-$(CONFIG_X86_32) += sys_i386_32.o i386_ksyms_32.o
obj-$(CONFIG_X86_64) += sys_x86_64.o x8664_ksyms_64.o
-obj-$(CONFIG_X86_64) += syscall_64.o vsyscall_64.o
+obj-$(CONFIG_X86_64) += syscall_64.o vsyscall_64.o vread_tsc_64.o
obj-y += bootflag.o e820.o
obj-y += pci-dma.o quirks.o topology.o kdebugfs.o
obj-y += alternative.o i8253.o pci-nommu.o hw_breakpoint.o
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 24249a5..6cc6922 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -763,40 +763,6 @@ static cycle_t read_tsc(struct clocksource *cs)
ret : clocksource_tsc.cycle_last;
}

-#ifdef CONFIG_X86_64
-static 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;
-}
-#endif
-
static void resume_tsc(struct clocksource *cs)
{
clocksource_tsc.cycle_last = 0;
diff --git a/arch/x86/kernel/vread_tsc_64.c b/arch/x86/kernel/vread_tsc_64.c
new file mode 100644
index 0000000..a81aa9e
--- /dev/null
+++ b/arch/x86/kernel/vread_tsc_64.c
@@ -0,0 +1,36 @@
+/* This code runs in userspace. */
+
+#define DISABLE_BRANCH_PROFILING
+#include <asm/vgtod.h>
+
+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;
+}
--
1.7.5.1


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