8512287a81
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>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
16 lines
248 B
ArmAsm
16 lines
248 B
ArmAsm
#include <linux/linkage.h>
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#include <asm/asm-offsets.h>
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.align 2
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.L_vdso_data_ptr:
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.long _start - . - VDSO_DATA_SIZE
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ENTRY(__get_datapage)
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.fnstart
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adr r0, .L_vdso_data_ptr
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ldr r1, [r0]
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add r0, r0, r1
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bx lr
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.fnend
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ENDPROC(__get_datapage)
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