[PATCH v11 0/4] ARM: VDSO

[PATCH v11 0/4] ARM: VDSO

[Nathan Lynch]

Provide fast userspace implementations of gettimeofday and
clock_gettime on systems that implement the generic timers extension
defined in ARMv7.  This follows the example of arm64 in conception but
significantly differs in some aspects of the implementation (C vs
assembly, mainly).

Clocks supported:
- CLOCK_REALTIME
- CLOCK_MONOTONIC
- CLOCK_REALTIME_COARSE
- CLOCK_MONOTONIC_COARSE

getcpu support is planned but not included at this time.

For applications to transparently benefit from this change,
ARM-specific support code needs to be added to glibc.  I hope to have
that code added to glibc for the 2.22 release, pending acceptance of
the kernel changes.

The VDSO symbols are available for lookup via dlsym even with an
unpatched glibc.

Here are some tables demonstrating the improvement on one platform
which benefits from the VDSO (OMAP5), and the hopefully acceptable
cost on two platforms lacking the architected timer.  The numbers are
the cost in nanoseconds for a call to gettimeofday via glibc, measured
with a microbenchmark.

| OMAP5           | glibc 2.21 | glibc 2.21 + |
| (Cortex-A15)    |            | vdso support |
|-----------------+------------+--------------|
| v4.0-rc1        |        346 |          351 |
| v4.0-rc1 + vdso |        343 |          118 |


| BeagleBone Black | glibc 2.21 | glibc 2.21 + |
| (Cortex-A8)      |            | vdso support |
|------------------+------------+--------------|
| v4.0-rc1         |        651 |          668 |
| v4.0-rc1 + vdso  |        651 |          661 |


| i.MX6Q          | glibc 2.21 | glibc 2.21 + |
| (Cortex-A9)     |            | vdso support |
|-----------------+------------+--------------|
| v4.0-rc1        |        563 |          579 |
| v4.0-rc1 + vdso |        569 |          584 |


These numbers vary slightly from one run to another (i.MX6Q in
particular seems a little "noisy" with variances of +/-10ns).

I believe this is the minimum overhead (or very close to it) that can
be achieved on systems lacking the arch timer, and it's due to adding
a test and branch to the syscall wrappers in glibc.  Previous versions
of these patches caused glibc to dispatch through the VDSO
unconditionally, imposing up to ~70ns penalty on i.MX6Q.  See patch #3
for how this has been addressed.

I've been verifying and benchmarking this with some custom test code
which I have hosted here:

https://github.com/nlynch-mentor/vdsotest

Unpatched FSF GDB may complain "warning: Could not load shared library
symbols for linux-vdso.so.1."  This is not an ARM-specific issue.
Current Fedora and Debian patch their GDB packages to prevent this
warning.

Changes since v10:
- Update to 4.0-rc1.
- Drop clock_getres support (easy to implement, but no use case).
- User access to virtual counter is always enabled since e1ce5c7adc73
  "clocksource: arm_arch_timer: Enable counter access for 32-bit ARM";
  drop runtime check.
- Zero out gettimeofday/clock_gettime entry points when the
  architected timer is unusable.
- Do not use the generic timer facility when the CPU registers are not
  initialized by firmware (arm,cpu-registers-not-fw-configured).
- Do not service coarse clocks if high-resolution clocks are not
  accelerated.

Changes since v9:
- Update to 3.17-rc5.
- Fixed minor checkpatch warnings (line length etc).
- Removed useless VDSO_LBASE constant.
- Added 'notrace' attributes to VDSO-internal functions.
- Removed unused debug code from vdsomunge host program.

Changes since v8:
- Update to 3.17-rc1.
- Split out arch timer changes into separate series.
- Ensure that VDSO will not attempt to read the counter if access is
  not enabled; this check can be removed after the arch timer changes
  are merged.  See update_vsyscall and vdso_can_use_arch_timer in
  patch #5.

Changes since v7:
- Update to next-20140801.
- In arch_setup_additional_pages, fix call to get_unmapped_area - use
  bytes (not pages) for length argument.
- As x86 does, separate data and text into two VMAs, [vvar] and [vdso]
  respectively.  These have different permissions; [vdso] will allow
  debuggers to set breakpoints, but [vvar] is read-only and cannot be
  modified even via ptrace.
- Use _install_special_mapping for signal page, vvar, and vdso
  mappings.
- Add -DDISABLE_BRANCH_PROFILING.
- Add --no-undefined -Bsymbolic to link options to cause linker to
  error out on unresolved references, making checkundef script
  unnecessary.
- Specify max-page-size, common-page-size in linker options so the
  true alignment is reflected in program header; otherwise gdb gets
  confused.
- Fix incremental build vs. generated auxvec.h.
- Use appropriate unwind directives in __get_datapage.
- Added vdso_install target and help text.  Install build-id symlinks
  as x86 does.
- Adjust update_vsyscall for changes in struct timekeeper.

Changes since v6:
- Update to 3.16-rc1.
- Remove -lgcc from link step - need to support GCC installations
  without libgcc.
- Force -O2 compilation to prevent GCC from emitting calls to libgcc
  math routines.
- Use custom post-processing to clear the EF_ARM_ABI_FLOAT_SOFT flag
  if set in the ELF header to produce a shared object which is
  architecturally allowed to be used by both soft- and hard-float
  code.
- Consolidate common arch timer code instead of duplicating it.
- Prevent the VDSO from attempting CP15 access on memory-only
  architected timer implementations by renaming the clocksource.

Changes since v5:
- Update to 3.15-rc1.
- Place vdso at a randomized offset above the stack along with the
  sigpage.
- Properly export asm/auxvec.h.
- Split patch into series for ease of review.

Changes since v4:
- Map data page at the beginning of the VMA to prevent orphan
  sections at the end of output invalidating the calculated offset.
- Move checkundef into cmd_vdsold to avoid spurious rebuilds.
- Change vdso_init message to pr_debug.
- Add -fno-stack-protector to cflags.

Changes since v3:
- Update to 3.14-rc6.
- Record vdso base in mm context before installing mapping (for the
  sake of perf_mmap_event).
- Use a more seqcount-like API for critical sections.  Using seqcount
  API directly, however, would leak kernel pointers to userspace when
  lockdep is enabled.
- Trap instead of looping forever in division-by-zero stubs.

Changes since v2:
- Update to 3.14-rc4.
- Make vDSO configurable, depending on AEABI and MMU.
- Defer shifting of nanosecond component of timespec: fixes observed
  1ns inconsistencies for CLOCK_REALTIME, CLOCK_MONOTONIC (see
  45a7905fc48f for arm64 equivalent).
- Force reload of seq_count when spinning: without a memory clobber
  after the load of vdata->seq_count, GCC can generate code like this:
    2f8:   e59c9020        ldr     r9, [ip, #32]
    2fc:   e3190001        tst     r9, #1
    300:   1a000033        bne     3d4 <do_realtime+0x104>
    304:   f57ff05b        dmb     ish
    308:   e59c3034        ldr     r3, [ip, #52]   ; 0x34
    ...
    3d4:   eafffffe        b       3d4 <do_realtime+0x104>
- Build vdso.so with -lgcc: calls to __lshrdi3, __divsi3 sometimes
  emitted (especially with -Os).  Override certain libgcc functions to
  prevent undefined symbols.
- Do not clear PG_reserved on vdso pages.
- Remove unnecessary get_page calls.
- Simplify ELF signature check during init.
- Use volatile for asm syscall fallbacks.
- Check whether vdso_pagelist is initialized in arm_install_vdso.
- Record clocksource mask in data page.
- Reduce code duplication in do_realtime, do_monotonic.
- Reduce calculations performed in critical sections.
- Simplify coarse clock handling.
- Move datapage load to its own assembly routine.
- Tune vdso_data layout and tweak field names.
- Check vdso shared object for undefined symbols during build.

Changes since v1:
- update to 3.14-rc1
- ensure cache coherency for data page
- Document the kernel-to-userspace protocol for vdso data page updates,
  and note that the timekeeping core prevents concurrent updates.
- update wall-to-monotonic fields unconditionally
- move vdso_start, vdso_end declarations to vdso.h
- correctly build and run when CONFIG_ARM_ARCH_TIMER=n
- rearrange linker script to avoid overlapping sections when CONFIG_DEBUGINFO=n
- remove use_syscall checks from coarse clock paths
- crib BUG_INSTR (0xe7f001f2) from asm/bug.h for text fill


Nathan Lynch (4):
  ARM: miscellaneous vdso infrastructure, preparation
  ARM: add VDSO user-space code
  ARM: VDSO initialization, mapping, and synchronization
  ARM: add CONFIG_VDSO Kconfig and Makefile bits

 arch/arm/Makefile                    |   8 +
 arch/arm/include/asm/Kbuild          |   1 -
 arch/arm/include/asm/auxvec.h        |   1 +
 arch/arm/include/asm/elf.h           |   9 +
 arch/arm/include/asm/mmu.h           |   3 +
 arch/arm/include/asm/vdso.h          |  32 ++++
 arch/arm/include/asm/vdso_datapage.h |  60 +++++++
 arch/arm/include/uapi/asm/Kbuild     |   1 +
 arch/arm/include/uapi/asm/auxvec.h   |   7 +
 arch/arm/kernel/Makefile             |   1 +
 arch/arm/kernel/asm-offsets.c        |   5 +
 arch/arm/kernel/process.c            |  17 +-
 arch/arm/kernel/vdso.c               | 337 +++++++++++++++++++++++++++++++++++
 arch/arm/mm/Kconfig                  |  14 ++
 arch/arm/vdso/.gitignore             |   1 +
 arch/arm/vdso/Makefile               |  74 ++++++++
 arch/arm/vdso/datapage.S             |  15 ++
 arch/arm/vdso/vdso.S                 |  35 ++++
 arch/arm/vdso/vdso.lds.S             |  87 +++++++++
 arch/arm/vdso/vdsomunge.c            | 201 +++++++++++++++++++++
 arch/arm/vdso/vgettimeofday.c        | 282 +++++++++++++++++++++++++++++
 21 files changed, 1187 insertions(+), 4 deletions(-)
 create mode 100644 arch/arm/include/asm/auxvec.h
 create mode 100644 arch/arm/include/asm/vdso.h
 create mode 100644 arch/arm/include/asm/vdso_datapage.h
 create mode 100644 arch/arm/include/uapi/asm/auxvec.h
 create mode 100644 arch/arm/kernel/vdso.c
 create mode 100644 arch/arm/vdso/.gitignore
 create mode 100644 arch/arm/vdso/Makefile
 create mode 100644 arch/arm/vdso/datapage.S
 create mode 100644 arch/arm/vdso/vdso.S
 create mode 100644 arch/arm/vdso/vdso.lds.S
 create mode 100644 arch/arm/vdso/vdsomunge.c
 create mode 100644 arch/arm/vdso/vgettimeofday.c

-- 
1.9.3

[PATCH v11 1/4] ARM: miscellaneous vdso infrastructure, preparation

[Nathan Lynch]

Define the layout of the data structure shared between kernel and
userspace.

Track the vdso address in the mm_context; needed for communicating
AT_SYSINFO_EHDR to the ELF loader.

Add declarations for arm_install_vdso; implementation is in a
following patch.

Define AT_SYSINFO_EHDR, and, if CONFIG_VDSO=y, report the vdso shared
object address via the ELF auxiliary vector.

Note - this adds the AT_SYSINFO_EHDR in a new user-visible header
asm/auxvec.h; this is consistent with other architectures.

Signed-off-by: Nathan Lynch <nathan_lynch@mentor.com>
---
 arch/arm/include/asm/Kbuild          |  1 -
 arch/arm/include/asm/auxvec.h        |  1 +
 arch/arm/include/asm/elf.h           |  9 ++++++
 arch/arm/include/asm/mmu.h           |  3 ++
 arch/arm/include/asm/vdso.h          | 32 +++++++++++++++++++
 arch/arm/include/asm/vdso_datapage.h | 60 ++++++++++++++++++++++++++++++++++++
 arch/arm/include/uapi/asm/Kbuild     |  1 +
 arch/arm/include/uapi/asm/auxvec.h   |  7 +++++
 8 files changed, 113 insertions(+), 1 deletion(-)
 create mode 100644 arch/arm/include/asm/auxvec.h
 create mode 100644 arch/arm/include/asm/vdso.h
 create mode 100644 arch/arm/include/asm/vdso_datapage.h
 create mode 100644 arch/arm/include/uapi/asm/auxvec.h

diff --git a/arch/arm/include/asm/Kbuild b/arch/arm/include/asm/Kbuild
index fe74c0d1e485..eb0f43f3e3f1 100644
--- a/arch/arm/include/asm/Kbuild
+++ b/arch/arm/include/asm/Kbuild
@@ -1,6 +1,5 @@
 
 
-generic-y += auxvec.h
 generic-y += bitsperlong.h
 generic-y += cputime.h
 generic-y += current.h
diff --git a/arch/arm/include/asm/auxvec.h b/arch/arm/include/asm/auxvec.h
new file mode 100644
index 000000000000..fbd388c46299
--- /dev/null
+++ b/arch/arm/include/asm/auxvec.h
@@ -0,0 +1 @@
+#include <uapi/asm/auxvec.h>
diff --git a/arch/arm/include/asm/elf.h b/arch/arm/include/asm/elf.h
index afb9cafd3786..ac3f17fb4c8d 100644
--- a/arch/arm/include/asm/elf.h
+++ b/arch/arm/include/asm/elf.h
@@ -1,7 +1,9 @@
 #ifndef __ASMARM_ELF_H
 #define __ASMARM_ELF_H
 
+#include <asm/auxvec.h>
 #include <asm/hwcap.h>
+#include <asm/vdso_datapage.h>
 
 /*
  * ELF register definitions..
@@ -130,6 +132,13 @@ extern unsigned long arch_randomize_brk(struct mm_struct *mm);
 #define arch_randomize_brk arch_randomize_brk
 
 #ifdef CONFIG_MMU
+#ifdef CONFIG_VDSO
+#define ARCH_DLINFO						\
+do {								\
+	NEW_AUX_ENT(AT_SYSINFO_EHDR,				\
+		    (elf_addr_t)current->mm->context.vdso);	\
+} while (0)
+#endif
 #define ARCH_HAS_SETUP_ADDITIONAL_PAGES 1
 struct linux_binprm;
 int arch_setup_additional_pages(struct linux_binprm *, int);
diff --git a/arch/arm/include/asm/mmu.h b/arch/arm/include/asm/mmu.h
index 64fd15159b7d..a5b47421059d 100644
--- a/arch/arm/include/asm/mmu.h
+++ b/arch/arm/include/asm/mmu.h
@@ -11,6 +11,9 @@ typedef struct {
 #endif
 	unsigned int	vmalloc_seq;
 	unsigned long	sigpage;
+#ifdef CONFIG_VDSO
+	unsigned long	vdso;
+#endif
 } mm_context_t;
 
 #ifdef CONFIG_CPU_HAS_ASID
diff --git a/arch/arm/include/asm/vdso.h b/arch/arm/include/asm/vdso.h
new file mode 100644
index 000000000000..d0295f1dd1a3
--- /dev/null
+++ b/arch/arm/include/asm/vdso.h
@@ -0,0 +1,32 @@
+#ifndef __ASM_VDSO_H
+#define __ASM_VDSO_H
+
+#ifdef __KERNEL__
+
+#ifndef __ASSEMBLY__
+
+struct mm_struct;
+
+#ifdef CONFIG_VDSO
+
+void arm_install_vdso(struct mm_struct *mm, unsigned long addr);
+
+extern char vdso_start, vdso_end;
+
+extern unsigned int vdso_total_pages;
+
+#else /* CONFIG_VDSO */
+
+static inline void arm_install_vdso(struct mm_struct *mm, unsigned long addr)
+{
+}
+
+#define vdso_total_pages 0
+
+#endif /* CONFIG_VDSO */
+
+#endif /* __ASSEMBLY__ */
+
+#endif /* __KERNEL__ */
+
+#endif /* __ASM_VDSO_H */
diff --git a/arch/arm/include/asm/vdso_datapage.h b/arch/arm/include/asm/vdso_datapage.h
new file mode 100644
index 000000000000..9be259442fca
--- /dev/null
+++ b/arch/arm/include/asm/vdso_datapage.h
@@ -0,0 +1,60 @@
+/*
+ * Adapted from arm64 version.
+ *
+ * Copyright (C) 2012 ARM Limited
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#ifndef __ASM_VDSO_DATAPAGE_H
+#define __ASM_VDSO_DATAPAGE_H
+
+#ifdef __KERNEL__
+
+#ifndef __ASSEMBLY__
+
+#include <asm/page.h>
+
+/* Try to be cache-friendly on systems that don't implement the
+ * generic timer: fit the unconditionally updated fields in the first
+ * 32 bytes.
+ */
+struct vdso_data {
+	u32 seq_count;		/* sequence count - odd during updates */
+	u16 tk_is_cntvct;	/* fall back to syscall if false */
+	u16 cs_shift;		/* clocksource shift */
+	u32 xtime_coarse_sec;	/* coarse time */
+	u32 xtime_coarse_nsec;
+
+	u32 wtm_clock_sec;	/* wall to monotonic offset */
+	u32 wtm_clock_nsec;
+	u32 xtime_clock_sec;	/* CLOCK_REALTIME - seconds */
+	u32 cs_mult;		/* clocksource multiplier */
+
+	u64 cs_cycle_last;	/* last cycle value */
+	u64 cs_mask;		/* clocksource mask */
+
+	u64 xtime_clock_snsec;	/* CLOCK_REALTIME sub-ns base */
+	u32 tz_minuteswest;	/* timezone info for gettimeofday(2) */
+	u32 tz_dsttime;
+};
+
+union vdso_data_store {
+	struct vdso_data data;
+	u8 page[PAGE_SIZE];
+};
+
+#endif /* !__ASSEMBLY__ */
+
+#endif /* __KERNEL__ */
+
+#endif /* __ASM_VDSO_DATAPAGE_H */
diff --git a/arch/arm/include/uapi/asm/Kbuild b/arch/arm/include/uapi/asm/Kbuild
index 70a1c9da30ca..a1c05f93d920 100644
--- a/arch/arm/include/uapi/asm/Kbuild
+++ b/arch/arm/include/uapi/asm/Kbuild
@@ -1,6 +1,7 @@
 # UAPI Header export list
 include include/uapi/asm-generic/Kbuild.asm
 
+header-y += auxvec.h
 header-y += byteorder.h
 header-y += fcntl.h
 header-y += hwcap.h
diff --git a/arch/arm/include/uapi/asm/auxvec.h b/arch/arm/include/uapi/asm/auxvec.h
new file mode 100644
index 000000000000..cb02a767a500
--- /dev/null
+++ b/arch/arm/include/uapi/asm/auxvec.h
@@ -0,0 +1,7 @@
+#ifndef __ASM_AUXVEC_H
+#define __ASM_AUXVEC_H
+
+/* VDSO location */
+#define AT_SYSINFO_EHDR	33
+
+#endif
-- 
1.9.3

[PATCH v11 2/4] ARM: add VDSO user-space code

[Nathan Lynch]

Place VDSO-related user-space code in arch/arm/kernel/vdso/.

It is almost completely written in C with some assembly helpers to
load the data page address, sample the counter, and fall back to
system calls when necessary.

The VDSO can service gettimeofday and clock_gettime when
CONFIG_ARM_ARCH_TIMER is enabled and the architected timer is present
(and correctly configured).  It reads the CP15-based virtual counter
to compute high-resolution timestamps.

Of particular note is that a post-processing step ("vdsomunge") is
necessary to produce a shared object which is architecturally allowed
to be used by both soft- and hard-float EABI programs.

The 2012 edition of the ARM ABI defines Tag_ABI_VFP_args = 3 "Code is
compatible with both the base and VFP variants; the user did not
permit non-variadic functions to pass FP parameters/results."
Unfortunately current toolchains do not support this tag, which is
ideally what we would use.

The best available option is to ensure that both EF_ARM_ABI_FLOAT_SOFT
and EF_ARM_ABI_FLOAT_HARD are unset in the ELF header's e_flags,
indicating that the shared object is "old" and should be accepted for
backward compatibility's sake.  While binutils < 2.24 appear to
produce a vdso.so with both flags clear, 2.24 always sets
EF_ARM_ABI_FLOAT_SOFT, with no way to inhibit this behavior.  So we
have to fix things up with a custom post-processing step.

In fact, the VDSO code in glibc does much less validation (including
checking these flags) than the code for handling conventional
file-backed shared libraries, so this is a bit moot unless glibc's
VDSO code becomes more strict.

Signed-off-by: Nathan Lynch <nathan_lynch@mentor.com>
---
 arch/arm/kernel/asm-offsets.c |   5 +
 arch/arm/vdso/.gitignore      |   1 +
 arch/arm/vdso/Makefile        |  74 +++++++++++
 arch/arm/vdso/datapage.S      |  15 +++
 arch/arm/vdso/vdso.S          |  35 ++++++
 arch/arm/vdso/vdso.lds.S      |  87 +++++++++++++
 arch/arm/vdso/vdsomunge.c     | 201 ++++++++++++++++++++++++++++++
 arch/arm/vdso/vgettimeofday.c | 282 ++++++++++++++++++++++++++++++++++++++++++
 8 files changed, 700 insertions(+)
 create mode 100644 arch/arm/vdso/.gitignore
 create mode 100644 arch/arm/vdso/Makefile
 create mode 100644 arch/arm/vdso/datapage.S
 create mode 100644 arch/arm/vdso/vdso.S
 create mode 100644 arch/arm/vdso/vdso.lds.S
 create mode 100644 arch/arm/vdso/vdsomunge.c
 create mode 100644 arch/arm/vdso/vgettimeofday.c

diff --git a/arch/arm/kernel/asm-offsets.c b/arch/arm/kernel/asm-offsets.c
index 2d2d6087b9b1..9147008f0d51 100644
--- a/arch/arm/kernel/asm-offsets.c
+++ b/arch/arm/kernel/asm-offsets.c
@@ -25,6 +25,7 @@
 #include <asm/memory.h>
 #include <asm/procinfo.h>
 #include <asm/suspend.h>
+#include <asm/vdso_datapage.h>
 #include <asm/hardware/cache-l2x0.h>
 #include <linux/kbuild.h>
 
@@ -210,5 +211,9 @@ int main(void)
 #endif
   DEFINE(KVM_VTTBR,		offsetof(struct kvm, arch.vttbr));
 #endif
+  BLANK();
+#ifdef CONFIG_VDSO
+  DEFINE(VDSO_DATA_SIZE,	sizeof(union vdso_data_store));
+#endif
   return 0; 
 }
diff --git a/arch/arm/vdso/.gitignore b/arch/arm/vdso/.gitignore
new file mode 100644
index 000000000000..f8b69d84238e
--- /dev/null
+++ b/arch/arm/vdso/.gitignore
@@ -0,0 +1 @@
+vdso.lds
diff --git a/arch/arm/vdso/Makefile b/arch/arm/vdso/Makefile
new file mode 100644
index 000000000000..bab0a8be7924
--- /dev/null
+++ b/arch/arm/vdso/Makefile
@@ -0,0 +1,74 @@
+hostprogs-y := vdsomunge
+
+obj-vdso := vgettimeofday.o datapage.o
+
+# Build rules
+targets := $(obj-vdso) vdso.so vdso.so.dbg vdso.so.raw vdso.lds
+obj-vdso := $(addprefix $(obj)/, $(obj-vdso))
+
+ccflags-y := -shared -fPIC -fno-common -fno-builtin -fno-stack-protector
+ccflags-y += -nostdlib -Wl,-soname=linux-vdso.so.1 -DDISABLE_BRANCH_PROFILING
+ccflags-y += -Wl,--no-undefined $(call cc-ldoption, -Wl$(comma)--hash-style=sysv)
+
+obj-y += vdso.o
+extra-y += vdso.lds
+CPPFLAGS_vdso.lds += -P -C -U$(ARCH)
+
+CFLAGS_REMOVE_vdso.o = -pg
+
+# Force -O2 to avoid libgcc dependencies
+CFLAGS_REMOVE_vgettimeofday.o = -pg -Os
+CFLAGS_vgettimeofday.o = -O2
+
+# Disable gcov profiling for VDSO code
+GCOV_PROFILE := n
+
+# Force dependency
+$(obj)/vdso.o : $(obj)/vdso.so
+
+# Link rule for the .so file
+$(obj)/vdso.so.raw: $(src)/vdso.lds $(obj-vdso) FORCE
+	$(call if_changed,vdsold)
+
+$(obj)/vdso.so.dbg: $(obj)/vdso.so.raw $(obj)/vdsomunge FORCE
+	$(call if_changed,vdsomunge)
+
+# Strip rule for the .so file
+$(obj)/%.so: OBJCOPYFLAGS := -S
+$(obj)/%.so: $(obj)/%.so.dbg FORCE
+	$(call if_changed,objcopy)
+
+# Actual build commands
+quiet_cmd_vdsold = VDSO    $@
+      cmd_vdsold = $(CC) $(c_flags) -Wl,-T $(filter %.lds,$^) $(filter %.o,$^) \
+                   $(call cc-ldoption, -Wl$(comma)--build-id) \
+                   -Wl,-Bsymbolic -Wl,-z,max-page-size=4096 \
+                   -Wl,-z,common-page-size=4096 -o $@
+
+quiet_cmd_vdsomunge = MUNGE   $@
+      cmd_vdsomunge = $(objtree)/$(obj)/vdsomunge $< $@
+
+#
+# Install the unstripped copy of vdso.so.dbg.  If our toolchain
+# supports build-id, install .build-id links as well.
+#
+# Cribbed from arch/x86/vdso/Makefile.
+#
+quiet_cmd_vdso_install = INSTALL $<
+define cmd_vdso_install
+	cp $< "$(MODLIB)/vdso/vdso.so"; \
+	if readelf -n $< | grep -q 'Build ID'; then \
+	  buildid=`readelf -n $< |grep 'Build ID' |sed -e 's/^.*Build ID: \(.*\)$$/\1/'`; \
+	  first=`echo $$buildid | cut -b-2`; \
+	  last=`echo $$buildid | cut -b3-`; \
+	  mkdir -p "$(MODLIB)/vdso/.build-id/$$first"; \
+	  ln -sf "../../vdso.so" "$(MODLIB)/vdso/.build-id/$$first/$$last.debug"; \
+	fi
+endef
+
+$(MODLIB)/vdso: FORCE
+	@mkdir -p $(MODLIB)/vdso
+
+PHONY += vdso_install
+vdso_install: $(obj)/vdso.so.dbg $(MODLIB)/vdso FORCE
+	$(call cmd,vdso_install)
diff --git a/arch/arm/vdso/datapage.S b/arch/arm/vdso/datapage.S
new file mode 100644
index 000000000000..a2e60367931b
--- /dev/null
+++ b/arch/arm/vdso/datapage.S
@@ -0,0 +1,15 @@
+#include <linux/linkage.h>
+#include <asm/asm-offsets.h>
+
+	.align 2
+.L_vdso_data_ptr:
+	.long	_start - . - VDSO_DATA_SIZE
+
+ENTRY(__get_datapage)
+	.fnstart
+	adr	r0, .L_vdso_data_ptr
+	ldr	r1, [r0]
+	add	r0, r0, r1
+	bx	lr
+	.fnend
+ENDPROC(__get_datapage)
diff --git a/arch/arm/vdso/vdso.S b/arch/arm/vdso/vdso.S
new file mode 100644
index 000000000000..b2b97e3e7bab
--- /dev/null
+++ b/arch/arm/vdso/vdso.S
@@ -0,0 +1,35 @@
+/*
+ * Adapted from arm64 version.
+ *
+ * Copyright (C) 2012 ARM Limited
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Author: Will Deacon <will.deacon@arm.com>
+ */
+
+#include <linux/init.h>
+#include <linux/linkage.h>
+#include <linux/const.h>
+#include <asm/page.h>
+
+	__PAGE_ALIGNED_DATA
+
+	.globl vdso_start, vdso_end
+	.balign PAGE_SIZE
+vdso_start:
+	.incbin "arch/arm/vdso/vdso.so"
+	.balign PAGE_SIZE
+vdso_end:
+
+	.previous
diff --git a/arch/arm/vdso/vdso.lds.S b/arch/arm/vdso/vdso.lds.S
new file mode 100644
index 000000000000..89ca89f12d23
--- /dev/null
+++ b/arch/arm/vdso/vdso.lds.S
@@ -0,0 +1,87 @@
+/*
+ * Adapted from arm64 version.
+ *
+ * GNU linker script for the VDSO library.
+ *
+ * Copyright (C) 2012 ARM Limited
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Author: Will Deacon <will.deacon@arm.com>
+ * Heavily based on the vDSO linker scripts for other archs.
+ */
+
+#include <linux/const.h>
+#include <asm/page.h>
+#include <asm/vdso.h>
+
+OUTPUT_FORMAT("elf32-littlearm", "elf32-bigarm", "elf32-littlearm")
+OUTPUT_ARCH(arm)
+
+SECTIONS
+{
+	PROVIDE(_start = .);
+
+	. = SIZEOF_HEADERS;
+
+	.hash		: { *(.hash) }			:text
+	.gnu.hash	: { *(.gnu.hash) }
+	.dynsym		: { *(.dynsym) }
+	.dynstr		: { *(.dynstr) }
+	.gnu.version	: { *(.gnu.version) }
+	.gnu.version_d	: { *(.gnu.version_d) }
+	.gnu.version_r	: { *(.gnu.version_r) }
+
+	.note		: { *(.note.*) }		:text	:note
+
+
+	.eh_frame_hdr	: { *(.eh_frame_hdr) }		:text	:eh_frame_hdr
+	.eh_frame	: { KEEP (*(.eh_frame)) }	:text
+
+	.dynamic	: { *(.dynamic) }		:text	:dynamic
+
+	.rodata		: { *(.rodata*) }		:text
+
+	.text		: { *(.text*) }			:text	=0xe7f001f2
+
+	.got		: { *(.got) }
+	.rel.plt	: { *(.rel.plt) }
+
+	/DISCARD/	: {
+		*(.note.GNU-stack)
+		*(.data .data.* .gnu.linkonce.d.* .sdata*)
+		*(.bss .sbss .dynbss .dynsbss)
+	}
+}
+
+/*
+ * We must supply the ELF program headers explicitly to get just one
+ * PT_LOAD segment, and set the flags explicitly to make segments read-only.
+ */
+PHDRS
+{
+	text		PT_LOAD		FLAGS(5) FILEHDR PHDRS; /* PF_R|PF_X */
+	dynamic		PT_DYNAMIC	FLAGS(4);		/* PF_R */
+	note		PT_NOTE		FLAGS(4);		/* PF_R */
+	eh_frame_hdr	PT_GNU_EH_FRAME;
+}
+
+VERSION
+{
+	LINUX_2.6 {
+	global:
+		__vdso_clock_gettime;
+		__vdso_gettimeofday;
+	local: *;
+	};
+}
diff --git a/arch/arm/vdso/vdsomunge.c b/arch/arm/vdso/vdsomunge.c
new file mode 100644
index 000000000000..9005b07296c8
--- /dev/null
+++ b/arch/arm/vdso/vdsomunge.c
@@ -0,0 +1,201 @@
+/*
+ * Copyright 2015 Mentor Graphics Corporation.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; version 2 of the
+ * License.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ *
+ * vdsomunge - Host program which produces a shared object
+ * architecturally specified to be usable by both soft- and hard-float
+ * programs.
+ *
+ * The Procedure Call Standard for the ARM Architecture (ARM IHI
+ * 0042E) says:
+ *
+ *	6.4.1 VFP and Base Standard Compatibility
+ *
+ *	Code compiled for the VFP calling standard is compatible with
+ *	the base standard (and vice-versa) if no floating-point or
+ *	containerized vector arguments or results are used.
+ *
+ * And ELF for the ARM Architecture (ARM IHI 0044E) (Table 4-2) says:
+ *
+ *	If both EF_ARM_ABI_FLOAT_XXXX bits are clear, conformance to the
+ *	base procedure-call standard is implied.
+ *
+ * The VDSO is built with -msoft-float, as with the rest of the ARM
+ * kernel, and uses no floating point arguments or results.  The build
+ * process will produce a shared object that may or may not have the
+ * EF_ARM_ABI_FLOAT_SOFT flag set (it seems to depend on the binutils
+ * version; binutils starting with 2.24 appears to set it).  The
+ * EF_ARM_ABI_FLOAT_HARD flag should definitely not be set, and this
+ * program will error out if it is.
+ *
+ * If the soft-float flag is set, this program clears it.  That's all
+ * it does.
+ */
+
+#define _GNU_SOURCE
+
+#include <byteswap.h>
+#include <elf.h>
+#include <errno.h>
+#include <error.h>
+#include <fcntl.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+#define HOST_ORDER ELFDATA2LSB
+#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+#define HOST_ORDER ELFDATA2MSB
+#endif
+
+/* Some of the ELF constants we'd like to use were added to <elf.h>
+ * relatively recently.
+ */
+#ifndef EF_ARM_EABI_VER5
+#define EF_ARM_EABI_VER5 0x05000000
+#endif
+
+#ifndef EF_ARM_ABI_FLOAT_SOFT
+#define EF_ARM_ABI_FLOAT_SOFT 0x200
+#endif
+
+#ifndef EF_ARM_ABI_FLOAT_HARD
+#define EF_ARM_ABI_FLOAT_HARD 0x400
+#endif
+
+static const char *outfile;
+
+static void cleanup(void)
+{
+	if (error_message_count > 0 && outfile != NULL)
+		unlink(outfile);
+}
+
+static Elf32_Word read_elf_word(Elf32_Word word, bool swap)
+{
+	return swap ? bswap_32(word) : word;
+}
+
+static Elf32_Half read_elf_half(Elf32_Half half, bool swap)
+{
+	return swap ? bswap_16(half) : half;
+}
+
+static void write_elf_word(Elf32_Word val, Elf32_Word *dst, bool swap)
+{
+	*dst = swap ? bswap_32(val) : val;
+}
+
+int main(int argc, char **argv)
+{
+	const Elf32_Ehdr *inhdr;
+	bool clear_soft_float;
+	const char *infile;
+	Elf32_Word e_flags;
+	const void *inbuf;
+	struct stat stat;
+	void *outbuf;
+	bool swap;
+	int outfd;
+	int infd;
+
+	atexit(cleanup);
+
+	if (argc != 3)
+		error(EXIT_FAILURE, 0, "Usage: %s [infile] [outfile]", argv[0]);
+
+	infile = argv[1];
+	outfile = argv[2];
+
+	infd = open(infile, O_RDONLY);
+	if (infd < 0)
+		error(EXIT_FAILURE, errno, "Cannot open %s", infile);
+
+	if (fstat(infd, &stat) != 0)
+		error(EXIT_FAILURE, errno, "Failed stat for %s", infile);
+
+	inbuf = mmap(NULL, stat.st_size, PROT_READ, MAP_PRIVATE, infd, 0);
+	if (inbuf == MAP_FAILED)
+		error(EXIT_FAILURE, errno, "Failed to map %s", infile);
+
+	close(infd);
+
+	inhdr = inbuf;
+
+	if (memcmp(&inhdr->e_ident, ELFMAG, SELFMAG) != 0)
+		error(EXIT_FAILURE, 0, "Not an ELF file");
+
+	if (inhdr->e_ident[EI_CLASS] != ELFCLASS32)
+		error(EXIT_FAILURE, 0, "Unsupported ELF class");
+
+	swap = inhdr->e_ident[EI_DATA] != HOST_ORDER;
+
+	if (read_elf_half(inhdr->e_type, swap) != ET_DYN)
+		error(EXIT_FAILURE, 0, "Not a shared object");
+
+	if (read_elf_half(inhdr->e_machine, swap) != EM_ARM) {
+		error(EXIT_FAILURE, 0, "Unsupported architecture %#x",
+		      inhdr->e_machine);
+	}
+
+	e_flags = read_elf_word(inhdr->e_flags, swap);
+
+	if (EF_ARM_EABI_VERSION(e_flags) != EF_ARM_EABI_VER5) {
+		error(EXIT_FAILURE, 0, "Unsupported EABI version %#x",
+		      EF_ARM_EABI_VERSION(e_flags));
+	}
+
+	if (e_flags & EF_ARM_ABI_FLOAT_HARD)
+		error(EXIT_FAILURE, 0,
+		      "Unexpected hard-float flag set in e_flags");
+
+	clear_soft_float = !!(e_flags & EF_ARM_ABI_FLOAT_SOFT);
+
+	outfd = open(outfile, O_RDWR | O_CREAT | O_TRUNC, S_IRUSR | S_IWUSR);
+	if (outfd < 0)
+		error(EXIT_FAILURE, errno, "Cannot open %s", outfile);
+
+	if (ftruncate(outfd, stat.st_size) != 0)
+		error(EXIT_FAILURE, errno, "Cannot truncate %s", outfile);
+
+	outbuf = mmap(NULL, stat.st_size, PROT_READ | PROT_WRITE, MAP_SHARED,
+		      outfd, 0);
+	if (outbuf == MAP_FAILED)
+		error(EXIT_FAILURE, errno, "Failed to map %s", outfile);
+
+	close(outfd);
+
+	memcpy(outbuf, inbuf, stat.st_size);
+
+	if (clear_soft_float) {
+		Elf32_Ehdr *outhdr;
+
+		outhdr = outbuf;
+		e_flags &= ~EF_ARM_ABI_FLOAT_SOFT;
+		write_elf_word(e_flags, &outhdr->e_flags, swap);
+	}
+
+	if (msync(outbuf, stat.st_size, MS_SYNC) != 0)
+		error(EXIT_FAILURE, errno, "Failed to sync %s", outfile);
+
+	return EXIT_SUCCESS;
+}
diff --git a/arch/arm/vdso/vgettimeofday.c b/arch/arm/vdso/vgettimeofday.c
new file mode 100644
index 000000000000..79214d5ff097
--- /dev/null
+++ b/arch/arm/vdso/vgettimeofday.c
@@ -0,0 +1,282 @@
+/*
+ * Copyright 2015 Mentor Graphics Corporation.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; version 2 of the
+ * License.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <linux/compiler.h>
+#include <linux/hrtimer.h>
+#include <linux/time.h>
+#include <asm/arch_timer.h>
+#include <asm/barrier.h>
+#include <asm/bug.h>
+#include <asm/page.h>
+#include <asm/unistd.h>
+#include <asm/vdso_datapage.h>
+
+#ifndef CONFIG_AEABI
+#error This code depends on AEABI system call conventions
+#endif
+
+extern struct vdso_data *__get_datapage(void);
+
+static notrace u32 __vdso_read_begin(const struct vdso_data *vdata)
+{
+	u32 seq;
+repeat:
+	seq = ACCESS_ONCE(vdata->seq_count);
+	if (seq & 1) {
+		cpu_relax();
+		goto repeat;
+	}
+	return seq;
+}
+
+static notrace u32 vdso_read_begin(const struct vdso_data *vdata)
+{
+	u32 seq;
+
+	seq = __vdso_read_begin(vdata);
+
+	smp_rmb(); /* Pairs with smp_wmb in vdso_write_end */
+	return seq;
+}
+
+static notrace int vdso_read_retry(const struct vdso_data *vdata, u32 start)
+{
+	smp_rmb(); /* Pairs with smp_wmb in vdso_write_begin */
+	return vdata->seq_count != start;
+}
+
+static notrace long clock_gettime_fallback(clockid_t _clkid,
+					   struct timespec *_ts)
+{
+	register struct timespec *ts asm("r1") = _ts;
+	register clockid_t clkid asm("r0") = _clkid;
+	register long ret asm ("r0");
+	register long nr asm("r7") = __NR_clock_gettime;
+
+	asm volatile(
+	"	swi #0\n"
+	: "=r" (ret)
+	: "r" (clkid), "r" (ts), "r" (nr)
+	: "memory");
+
+	return ret;
+}
+
+static notrace int do_realtime_coarse(struct timespec *ts,
+				      struct vdso_data *vdata)
+{
+	u32 seq;
+
+	do {
+		seq = vdso_read_begin(vdata);
+
+		ts->tv_sec = vdata->xtime_coarse_sec;
+		ts->tv_nsec = vdata->xtime_coarse_nsec;
+
+	} while (vdso_read_retry(vdata, seq));
+
+	return 0;
+}
+
+static notrace int do_monotonic_coarse(struct timespec *ts,
+				       struct vdso_data *vdata)
+{
+	struct timespec tomono;
+	u32 seq;
+
+	do {
+		seq = vdso_read_begin(vdata);
+
+		ts->tv_sec = vdata->xtime_coarse_sec;
+		ts->tv_nsec = vdata->xtime_coarse_nsec;
+
+		tomono.tv_sec = vdata->wtm_clock_sec;
+		tomono.tv_nsec = vdata->wtm_clock_nsec;
+
+	} while (vdso_read_retry(vdata, seq));
+
+	ts->tv_sec += tomono.tv_sec;
+	timespec_add_ns(ts, tomono.tv_nsec);
+
+	return 0;
+}
+
+#ifdef CONFIG_ARM_ARCH_TIMER
+
+static notrace u64 get_ns(struct vdso_data *vdata)
+{
+	u64 cycle_delta;
+	u64 cycle_now;
+	u64 nsec;
+
+	cycle_now = arch_counter_get_cntvct();
+
+	cycle_delta = (cycle_now - vdata->cs_cycle_last) & vdata->cs_mask;
+
+	nsec = (cycle_delta * vdata->cs_mult) + vdata->xtime_clock_snsec;
+	nsec >>= vdata->cs_shift;
+
+	return nsec;
+}
+
+static notrace int do_realtime(struct timespec *ts, struct vdso_data *vdata)
+{
+	u64 nsecs;
+	u32 seq;
+
+	do {
+		seq = vdso_read_begin(vdata);
+
+		if (!vdata->tk_is_cntvct)
+			return -1;
+
+		ts->tv_sec = vdata->xtime_clock_sec;
+		nsecs = get_ns(vdata);
+
+	} while (vdso_read_retry(vdata, seq));
+
+	ts->tv_nsec = 0;
+	timespec_add_ns(ts, nsecs);
+
+	return 0;
+}
+
+static notrace int do_monotonic(struct timespec *ts, struct vdso_data *vdata)
+{
+	struct timespec tomono;
+	u64 nsecs;
+	u32 seq;
+
+	do {
+		seq = vdso_read_begin(vdata);
+
+		if (!vdata->tk_is_cntvct)
+			return -1;
+
+		ts->tv_sec = vdata->xtime_clock_sec;
+		nsecs = get_ns(vdata);
+
+		tomono.tv_sec = vdata->wtm_clock_sec;
+		tomono.tv_nsec = vdata->wtm_clock_nsec;
+
+	} while (vdso_read_retry(vdata, seq));
+
+	ts->tv_sec += tomono.tv_sec;
+	ts->tv_nsec = 0;
+	timespec_add_ns(ts, nsecs + tomono.tv_nsec);
+
+	return 0;
+}
+
+#else /* CONFIG_ARM_ARCH_TIMER */
+
+static notrace int do_realtime(struct timespec *ts, struct vdso_data *vdata)
+{
+	return -1;
+}
+
+static notrace int do_monotonic(struct timespec *ts, struct vdso_data *vdata)
+{
+	return -1;
+}
+
+#endif /* CONFIG_ARM_ARCH_TIMER */
+
+notrace int __vdso_clock_gettime(clockid_t clkid, struct timespec *ts)
+{
+	struct vdso_data *vdata;
+	int ret = -1;
+
+	vdata = __get_datapage();
+
+	switch (clkid) {
+	case CLOCK_REALTIME_COARSE:
+		ret = do_realtime_coarse(ts, vdata);
+		break;
+	case CLOCK_MONOTONIC_COARSE:
+		ret = do_monotonic_coarse(ts, vdata);
+		break;
+	case CLOCK_REALTIME:
+		ret = do_realtime(ts, vdata);
+		break;
+	case CLOCK_MONOTONIC:
+		ret = do_monotonic(ts, vdata);
+		break;
+	default:
+		break;
+	}
+
+	if (ret)
+		ret = clock_gettime_fallback(clkid, ts);
+
+	return ret;
+}
+
+static notrace long gettimeofday_fallback(struct timeval *_tv,
+					  struct timezone *_tz)
+{
+	register struct timezone *tz asm("r1") = _tz;
+	register struct timeval *tv asm("r0") = _tv;
+	register long ret asm ("r0");
+	register long nr asm("r7") = __NR_gettimeofday;
+
+	asm volatile(
+	"	swi #0\n"
+	: "=r" (ret)
+	: "r" (tv), "r" (tz), "r" (nr)
+	: "memory");
+
+	return ret;
+}
+
+notrace int __vdso_gettimeofday(struct timeval *tv, struct timezone *tz)
+{
+	struct timespec ts;
+	struct vdso_data *vdata;
+	int ret;
+
+	vdata = __get_datapage();
+
+	ret = do_realtime(&ts, vdata);
+	if (ret)
+		return gettimeofday_fallback(tv, tz);
+
+	if (tv) {
+		tv->tv_sec = ts.tv_sec;
+		tv->tv_usec = ts.tv_nsec / 1000;
+	}
+	if (tz) {
+		tz->tz_minuteswest = vdata->tz_minuteswest;
+		tz->tz_dsttime = vdata->tz_dsttime;
+	}
+
+	return ret;
+}
+
+/* Avoid unresolved references emitted by GCC */
+
+void __aeabi_unwind_cpp_pr0(void)
+{
+}
+
+void __aeabi_unwind_cpp_pr1(void)
+{
+}
+
+void __aeabi_unwind_cpp_pr2(void)
+{
+}
-- 
1.9.3

[PATCH v11 3/4] ARM: VDSO initialization, mapping, and synchronization

[Nathan Lynch]

Initialize the VDSO page list at boot, install the VDSO mapping at
exec time, and update the data page during timer ticks.  This code is
not built if CONFIG_VDSO is not enabled.

Account for the VDSO length when randomizing the offset from the
stack.  The [vdso] and [vvar] pages are placed immediately following
the sigpage with separate _install_special_mapping calls.

We want to "penalize" systems lacking the arch timer as little
as possible.  Previous versions of this code installed the VDSO
unconditionally and unmodified, making it a measurably slower way for
glibc to invoke the real syscalls on such systems.  E.g. calling
gettimeofday via glibc goes from ~560ns to ~630ns on i.MX6Q.

If we can indicate to glibc that the time-related APIs in the VDSO are
not accelerated, glibc can continue to invoke the syscalls directly
instead of dispatching through the VDSO only to fall back to the slow
path.

Thus, if the architected timer is unusable for whatever reason, patch
the VDSO at boot time so that symbol lookups for gettimeofday and
clock_gettime return NULL.  (This is similar to what powerpc does and
borrows code from there.)  This allows glibc to perform the syscall
directly instead of passing control to the VDSO, which minimizes the
penalty.  In my measurements the time taken for a gettimeofday call
via glibc goes from ~560ns to ~580ns (again on i.MX6Q), and this is
solely due to adding a test and branch to glibc's gettimeofday syscall
wrapper.

An alternative to patching the VDSO at boot would be to not install
the VDSO at all when the arch timer isn't usable.  Another alternative
is to include a separate "dummy" vdso.so without gettimeofday and
clock_gettime, which would be selected at boot time.  Either of these
would get cumbersome if the VDSO were to gain support for an API such
as getcpu which is unrelated to arch timer support.

Signed-off-by: Nathan Lynch <nathan_lynch@mentor.com>
---
 arch/arm/kernel/process.c |  17 ++-
 arch/arm/kernel/vdso.c    | 337 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 351 insertions(+), 3 deletions(-)
 create mode 100644 arch/arm/kernel/vdso.c

diff --git a/arch/arm/kernel/process.c b/arch/arm/kernel/process.c
index fdfa3a78ec8c..c50fe212fd89 100644
--- a/arch/arm/kernel/process.c
+++ b/arch/arm/kernel/process.c
@@ -41,6 +41,7 @@
 #include <asm/system_misc.h>
 #include <asm/mach/time.h>
 #include <asm/tls.h>
+#include <asm/vdso.h>
 
 #ifdef CONFIG_CC_STACKPROTECTOR
 #include <linux/stackprotector.h>
@@ -475,7 +476,7 @@ const char *arch_vma_name(struct vm_area_struct *vma)
 }
 
 /* If possible, provide a placement hint at a random offset from the
- * stack for the signal page.
+ * stack for the sigpage and vdso pages.
  */
 static unsigned long sigpage_addr(const struct mm_struct *mm,
 				  unsigned int npages)
@@ -519,6 +520,7 @@ int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
 {
 	struct mm_struct *mm = current->mm;
 	struct vm_area_struct *vma;
+	unsigned long npages;
 	unsigned long addr;
 	unsigned long hint;
 	int ret = 0;
@@ -528,9 +530,12 @@ int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
 	if (!signal_page)
 		return -ENOMEM;
 
+	npages = 1; /* for sigpage */
+	npages += vdso_total_pages;
+
 	down_write(&mm->mmap_sem);
-	hint = sigpage_addr(mm, 1);
-	addr = get_unmapped_area(NULL, hint, PAGE_SIZE, 0, 0);
+	hint = sigpage_addr(mm, npages);
+	addr = get_unmapped_area(NULL, hint, npages << PAGE_SHIFT, 0, 0);
 	if (IS_ERR_VALUE(addr)) {
 		ret = addr;
 		goto up_fail;
@@ -547,6 +552,12 @@ int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
 
 	mm->context.sigpage = addr;
 
+	/* Unlike the sigpage, failure to install the vdso is unlikely
+	 * to be fatal to the process, so no error check needed
+	 * here.
+	 */
+	arm_install_vdso(mm, addr + PAGE_SIZE);
+
  up_fail:
 	up_write(&mm->mmap_sem);
 	return ret;
diff --git a/arch/arm/kernel/vdso.c b/arch/arm/kernel/vdso.c
new file mode 100644
index 000000000000..0d31d3ccab81
--- /dev/null
+++ b/arch/arm/kernel/vdso.c
@@ -0,0 +1,337 @@
+/*
+ * Adapted from arm64 version.
+ *
+ * Copyright (C) 2012 ARM Limited
+ * Copyright (C) 2015 Mentor Graphics Corporation.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <linux/elf.h>
+#include <linux/err.h>
+#include <linux/kernel.h>
+#include <linux/mm.h>
+#include <linux/of.h>
+#include <linux/printk.h>
+#include <linux/slab.h>
+#include <linux/timekeeper_internal.h>
+#include <linux/vmalloc.h>
+#include <asm/arch_timer.h>
+#include <asm/barrier.h>
+#include <asm/cacheflush.h>
+#include <asm/page.h>
+#include <asm/vdso.h>
+#include <asm/vdso_datapage.h>
+#include <clocksource/arm_arch_timer.h>
+
+#define MAX_SYMNAME	64
+
+static struct page **vdso_text_pagelist;
+
+/* Total number of pages needed for the data and text portions of the VDSO. */
+unsigned int vdso_total_pages __read_mostly;
+
+/*
+ * The VDSO data page.
+ */
+static union vdso_data_store vdso_data_store __page_aligned_data;
+static struct vdso_data *vdso_data = &vdso_data_store.data;
+
+static struct page *vdso_data_page;
+static struct vm_special_mapping vdso_data_mapping = {
+	.name = "[vvar]",
+	.pages = &vdso_data_page,
+};
+
+static struct vm_special_mapping vdso_text_mapping = {
+	.name = "[vdso]",
+};
+
+struct elfinfo {
+	Elf32_Ehdr	*hdr;		/* ptr to ELF */
+	Elf32_Sym	*dynsym;	/* ptr to .dynsym section */
+	unsigned long	dynsymsize;	/* size of .dynsym section */
+	char		*dynstr;	/* ptr to .dynstr section */
+};
+
+/* Cached result of boot-time check for whether the arch timer exists,
+ * and if so, whether the virtual counter is useable.
+ */
+static bool cntvct_ok __read_mostly;
+
+static bool __init cntvct_functional(void)
+{
+	struct device_node *np;
+	bool ret = false;
+
+	if (!IS_ENABLED(CONFIG_ARM_ARCH_TIMER))
+		goto out;
+
+	/* The arm_arch_timer core should export
+	 * arch_timer_use_virtual or similar so we don't have to do
+	 * this.
+	 */
+	np = of_find_compatible_node(NULL, NULL, "arm,armv7-timer");
+	if (!np)
+		goto out_put;
+
+	if (of_property_read_bool(np, "arm,cpu-registers-not-fw-configured"))
+		goto out_put;
+
+	ret = true;
+
+out_put:
+	of_node_put(np);
+out:
+	return ret;
+}
+
+static void * __init find_section(Elf32_Ehdr *ehdr, const char *name,
+				  unsigned long *size)
+{
+	Elf32_Shdr *sechdrs;
+	unsigned int i;
+	char *secnames;
+
+	/* Grab section headers and strings so we can tell who is who */
+	sechdrs = (void *)ehdr + ehdr->e_shoff;
+	secnames = (void *)ehdr + sechdrs[ehdr->e_shstrndx].sh_offset;
+
+	/* Find the section they want */
+	for (i = 1; i < ehdr->e_shnum; i++) {
+		if (strcmp(secnames + sechdrs[i].sh_name, name) == 0) {
+			if (size)
+				*size = sechdrs[i].sh_size;
+			return (void *)ehdr + sechdrs[i].sh_offset;
+		}
+	}
+
+	if (size)
+		*size = 0;
+	return NULL;
+}
+
+static Elf32_Sym * __init find_symbol(struct elfinfo *lib, const char *symname)
+{
+	unsigned int i;
+
+	for (i = 0; i < (lib->dynsymsize / sizeof(Elf32_Sym)); i++) {
+		char name[MAX_SYMNAME], *c;
+
+		if (lib->dynsym[i].st_name == 0)
+			continue;
+		strlcpy(name, lib->dynstr + lib->dynsym[i].st_name,
+			MAX_SYMNAME);
+		c = strchr(name, '@');
+		if (c)
+			*c = 0;
+		if (strcmp(symname, name) == 0)
+			return &lib->dynsym[i];
+	}
+	return NULL;
+}
+
+static void __init vdso_nullpatch_one(struct elfinfo *lib, const char *symname)
+{
+	Elf32_Sym *sym;
+
+	sym = find_symbol(lib, symname);
+	if (!sym)
+		return;
+
+	sym->st_name = 0;
+}
+
+static void __init patch_vdso(void *ehdr)
+{
+	struct elfinfo einfo;
+
+	einfo = (struct elfinfo) {
+		.hdr = ehdr,
+	};
+
+	einfo.dynsym = find_section(einfo.hdr, ".dynsym", &einfo.dynsymsize);
+	einfo.dynstr = find_section(einfo.hdr, ".dynstr", NULL);
+
+	/* If the virtual counter is absent or non-functional we don't
+	 * want programs to incur the slight additional overhead of
+	 * dispatching through the VDSO only to fall back to syscalls.
+	 */
+	if (!cntvct_ok) {
+		vdso_nullpatch_one(&einfo, "__vdso_gettimeofday");
+		vdso_nullpatch_one(&einfo, "__vdso_clock_gettime");
+	}
+}
+
+static int __init vdso_init(void)
+{
+	unsigned int text_pages;
+	int i;
+
+	if (memcmp(&vdso_start, "\177ELF", 4)) {
+		pr_err("VDSO is not a valid ELF object!\n");
+		return -ENOEXEC;
+	}
+
+	text_pages = (&vdso_end - &vdso_start) >> PAGE_SHIFT;
+	pr_debug("vdso: %i text pages at base %p\n", text_pages, &vdso_start);
+
+	/* Allocate the VDSO text pagelist */
+	vdso_text_pagelist = kcalloc(text_pages, sizeof(struct page *),
+				     GFP_KERNEL);
+	if (vdso_text_pagelist == NULL)
+		return -ENOMEM;
+
+	/* Grab the VDSO data page. */
+	vdso_data_page = virt_to_page(vdso_data);
+
+	/* Grab the VDSO text pages. */
+	for (i = 0; i < text_pages; i++) {
+		struct page *page;
+
+		page = virt_to_page(&vdso_start + i * PAGE_SIZE);
+		vdso_text_pagelist[i] = page;
+	}
+
+	vdso_text_mapping.pages = vdso_text_pagelist;
+
+	vdso_total_pages = 1; /* for the data/vvar page */
+	vdso_total_pages += text_pages;
+
+	cntvct_ok = cntvct_functional();
+
+	patch_vdso(&vdso_start);
+
+	return 0;
+}
+arch_initcall(vdso_init);
+
+static int install_vvar(struct mm_struct *mm, unsigned long addr)
+{
+	struct vm_area_struct *vma;
+
+	vma = _install_special_mapping(mm, addr, PAGE_SIZE,
+				       VM_READ | VM_MAYREAD,
+				       &vdso_data_mapping);
+
+	return IS_ERR(vma) ? PTR_ERR(vma) : 0;
+}
+
+/* assumes mmap_sem is write-locked */
+void arm_install_vdso(struct mm_struct *mm, unsigned long addr)
+{
+	struct vm_area_struct *vma;
+	unsigned long len;
+
+	mm->context.vdso = 0;
+
+	if (vdso_text_pagelist == NULL)
+		return;
+
+	if (install_vvar(mm, addr))
+		return;
+
+	/* Account for vvar page. */
+	addr += PAGE_SIZE;
+	len = (vdso_total_pages - 1) << PAGE_SHIFT;
+
+	vma = _install_special_mapping(mm, addr, len,
+		VM_READ | VM_EXEC | VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC,
+		&vdso_text_mapping);
+
+	if (!IS_ERR(vma))
+		mm->context.vdso = addr;
+}
+
+static void vdso_write_begin(struct vdso_data *vdata)
+{
+	++vdso_data->seq_count;
+	smp_wmb(); /* Pairs with smp_rmb in vdso_read_retry */
+}
+
+static void vdso_write_end(struct vdso_data *vdata)
+{
+	smp_wmb(); /* Pairs with smp_rmb in vdso_read_begin */
+	++vdso_data->seq_count;
+}
+
+static bool tk_is_cntvct(const struct timekeeper *tk)
+{
+	if (!IS_ENABLED(CONFIG_ARM_ARCH_TIMER))
+		return false;
+
+	if (strcmp(tk->tkr.clock->name, "arch_sys_counter") != 0)
+		return false;
+
+	return true;
+}
+
+/**
+ * update_vsyscall - update the vdso data page
+ *
+ * Increment the sequence counter, making it odd, indicating to
+ * userspace that an update is in progress.  Update the fields used
+ * for coarse clocks and, if the architected system timer is in use,
+ * the fields used for high precision clocks.  Increment the sequence
+ * counter again, making it even, indicating to userspace that the
+ * update is finished.
+ *
+ * Userspace is expected to sample seq_count before reading any other
+ * fields from the data page.  If seq_count is odd, userspace is
+ * expected to wait until it becomes even.  After copying data from
+ * the page, userspace must sample seq_count again; if it has changed
+ * from its previous value, userspace must retry the whole sequence.
+ *
+ * Calls to update_vsyscall are serialized by the timekeeping core.
+ */
+void update_vsyscall(struct timekeeper *tk)
+{
+	struct timespec xtime_coarse;
+	struct timespec64 *wtm = &tk->wall_to_monotonic;
+
+	if (!cntvct_ok) {
+		/* The entry points have been zeroed, so there is no
+		 * point in updating the data page.
+		 */
+		return;
+	}
+
+	vdso_write_begin(vdso_data);
+
+	xtime_coarse = __current_kernel_time();
+	vdso_data->tk_is_cntvct			= tk_is_cntvct(tk);
+	vdso_data->xtime_coarse_sec		= xtime_coarse.tv_sec;
+	vdso_data->xtime_coarse_nsec		= xtime_coarse.tv_nsec;
+	vdso_data->wtm_clock_sec		= wtm->tv_sec;
+	vdso_data->wtm_clock_nsec		= wtm->tv_nsec;
+
+	if (vdso_data->tk_is_cntvct) {
+		vdso_data->cs_cycle_last	= tk->tkr.cycle_last;
+		vdso_data->xtime_clock_sec	= tk->xtime_sec;
+		vdso_data->xtime_clock_snsec	= tk->tkr.xtime_nsec;
+		vdso_data->cs_mult		= tk->tkr.mult;
+		vdso_data->cs_shift		= tk->tkr.shift;
+		vdso_data->cs_mask		= tk->tkr.mask;
+	}
+
+	vdso_write_end(vdso_data);
+
+	flush_dcache_page(virt_to_page(vdso_data));
+}
+
+void update_vsyscall_tz(void)
+{
+	vdso_data->tz_minuteswest	= sys_tz.tz_minuteswest;
+	vdso_data->tz_dsttime		= sys_tz.tz_dsttime;
+	flush_dcache_page(virt_to_page(vdso_data));
+}
-- 
1.9.3

[PATCH v11 4/4] ARM: add CONFIG_VDSO Kconfig and Makefile bits

[Nathan Lynch]

Allow users to enable the vdso in Kconfig; include the vdso in the
build if CONFIG_VDSO is enabled.  Add 'vdso_install' target.

Signed-off-by: Nathan Lynch <nathan_lynch@mentor.com>
---
 arch/arm/Makefile        |  8 ++++++++
 arch/arm/kernel/Makefile |  1 +
 arch/arm/mm/Kconfig      | 14 ++++++++++++++
 3 files changed, 23 insertions(+)

diff --git a/arch/arm/Makefile b/arch/arm/Makefile
index 7f99cd652203..6c13a84b6cd2 100644
--- a/arch/arm/Makefile
+++ b/arch/arm/Makefile
@@ -263,6 +263,7 @@ core-$(CONFIG_FPE_FASTFPE)	+= $(FASTFPE_OBJ)
 core-$(CONFIG_VFP)		+= arch/arm/vfp/
 core-$(CONFIG_XEN)		+= arch/arm/xen/
 core-$(CONFIG_KVM_ARM_HOST) 	+= arch/arm/kvm/
+core-$(CONFIG_VDSO)		+= arch/arm/vdso/
 
 # If we have a machine-specific directory, then include it in the build.
 core-y				+= arch/arm/kernel/ arch/arm/mm/ arch/arm/common/
@@ -320,6 +321,12 @@ dtbs: prepare scripts
 dtbs_install:
 	$(Q)$(MAKE) $(dtbinst)=$(boot)/dts
 
+PHONY += vdso_install
+vdso_install:
+ifeq ($(CONFIG_VDSO),y)
+	$(Q)$(MAKE) $(build)=arch/arm/vdso $@
+endif
+
 # We use MRPROPER_FILES and CLEAN_FILES now
 archclean:
 	$(Q)$(MAKE) $(clean)=$(boot)
@@ -344,4 +351,5 @@ define archhelp
   echo  '                  Install using (your) ~/bin/$(INSTALLKERNEL) or'
   echo  '                  (distribution) /sbin/$(INSTALLKERNEL) or'
   echo  '                  install to $$(INSTALL_PATH) and run lilo'
+  echo  '  vdso_install  - Install unstripped vdso.so to $$(INSTALL_MOD_PATH)/vdso'
 endef
diff --git a/arch/arm/kernel/Makefile b/arch/arm/kernel/Makefile
index 902397dd1000..3e316ca54e40 100644
--- a/arch/arm/kernel/Makefile
+++ b/arch/arm/kernel/Makefile
@@ -75,6 +75,7 @@ obj-$(CONFIG_HW_PERF_EVENTS)	+= perf_event.o perf_event_cpu.o
 CFLAGS_pj4-cp0.o		:= -marm
 AFLAGS_iwmmxt.o			:= -Wa,-mcpu=iwmmxt
 obj-$(CONFIG_ARM_CPU_TOPOLOGY)  += topology.o
+obj-$(CONFIG_VDSO)		+= vdso.o
 
 ifneq ($(CONFIG_ARCH_EBSA110),y)
   obj-y		+= io.o
diff --git a/arch/arm/mm/Kconfig b/arch/arm/mm/Kconfig
index 9b4f29e595a4..8a5f1e644104 100644
--- a/arch/arm/mm/Kconfig
+++ b/arch/arm/mm/Kconfig
@@ -825,6 +825,20 @@ config KUSER_HELPERS
 	  Say N here only if you are absolutely certain that you do not
 	  need these helpers; otherwise, the safe option is to say Y.
 
+config VDSO
+	bool "Enable VDSO for acceleration of some system calls"
+	depends on AEABI && MMU
+	default y if ARM_ARCH_TIMER
+	select GENERIC_TIME_VSYSCALL
+	help
+	  Place in the process address space an ELF shared object
+	  providing fast implementations of gettimeofday and
+	  clock_gettime.  Systems that implement the ARM architected
+	  timer will receive maximum benefit.
+
+	  You must have glibc 2.22 or later for programs to seamlessly
+	  take advantage of this.
+
 config DMA_CACHE_RWFO
 	bool "Enable read/write for ownership DMA cache maintenance"
 	depends on CPU_V6K && SMP
-- 
1.9.3

[PATCH v11 0/4] ARM: VDSO

[Russell King - ARM Linux]

On Fri, Mar 20, 2015 at 04:13:19PM -0500, Nathan Lynch wrote:
> Provide fast userspace implementations of gettimeofday and
> clock_gettime on systems that implement the generic timers extension
> defined in ARMv7.  This follows the example of arm64 in conception but
> significantly differs in some aspects of the implementation (C vs
> assembly, mainly).

Okay, having re-read the code, I'm happy with this now.  I'll see about
testing it out on the platforms I have here in the next few days.  As
I now have a Cortex A15 platform, I can hopefully also check what effect
it has there (assuming ethernet gets fixed on it so I can get programs
onto the platform.)

-- 
FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up
according to speedtest.net.

[PATCH v11 0/4] ARM: VDSO

[Nathan Lynch]

On 03/25/2015 05:10 AM, Russell King - ARM Linux wrote:
> On Fri, Mar 20, 2015 at 04:13:19PM -0500, Nathan Lynch wrote:
>> Provide fast userspace implementations of gettimeofday and
>> clock_gettime on systems that implement the generic timers extension
>> defined in ARMv7.  This follows the example of arm64 in conception but
>> significantly differs in some aspects of the implementation (C vs
>> assembly, mainly).
> 
> Okay, having re-read the code, I'm happy with this now.  I'll see about
> testing it out on the platforms I have here in the next few days.  As
> I now have a Cortex A15 platform, I can hopefully also check what effect
> it has there (assuming ethernet gets fixed on it so I can get programs
> onto the platform.)

Thanks Russell, I'll put it in the patch tracker today.

I'll follow up with a link to the glibc patch when I post that too.

[PATCH v11 0/4] ARM: VDSO

[Nathan Lynch]

Patch for glibc posted here:

https://sourceware.org/ml/libc-alpha/2015-04/msg00059.html