When CONFIG_VDSO=n, the build normally does not enter arch/arm/vdso/
because arch/arm/Makefile does not add it to core-y.
However, if the user runs 'make arch/arm/vdso/' the VDSO targets will
get visited. This is because the VDSO Makefile itself does not
consider the value of CONFIG_VDSO.
It is arguably better and more consistent behavior to generate an
empty built-in.o when CONFIG_VDSO=n and the user attempts to build
arch/arm/vdso/. It's nicer because it doesn't try to build things
that Kconfig dependencies are there to prevent (e.g. the dependency on
AEABI), and it's less confusing than building objects that won't be
used in the final image.
Signed-off-by: Nathan Lynch <nathan_lynch@mentor.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
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>
Nathan Lynch