• Nathan Lynch's avatar
    ARM: 8330/1: add VDSO user-space code · 8512287a
    Nathan Lynch authored
    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: default avatarNathan Lynch <nathan_lynch@mentor.com>
    Signed-off-by: default avatarRussell King <rmk+kernel@arm.linux.org.uk>
    8512287a
vdso.lds.S 2.08 KB