• Nicolas Pitre's avatar
    [PATCH] ARM: 2651/3: kernel helpers for NPTL support · 2d2669b6
    Nicolas Pitre authored
    Patch from Nicolas Pitre
    
    This patch entirely reworks the kernel assistance for NPTL on ARM.
    In particular this provides an efficient way to retrieve the TLS
    value and perform atomic operations without any instruction emulation
    nor special system call.  This even allows for pre ARMv6 binaries to
    be forward compatible with SMP systems without any penalty.
    The problematic and performance critical operations are performed
    through segment of kernel provided user code reachable from user space
    at a fixed address in kernel memory.  Those fixed entry points are
    within the vector page so we basically get it for free as no extra
    memory page is required and nothing else may be mapped at that
    location anyway.
    This is different from (but doesn't preclude) a full blown VDSO
    implementation, however a VDSO would prevent some assembly tricks with
    constants that allows for efficient branching to those code segments.
    And since those code segments only use a few cycles before returning to
    user code, the overhead of a VDSO far call would add a significant
    overhead to such minimalistic operations.
    The ARM_NR_set_tls syscall also changed number.  This is done for two
    reasons:
    1) this patch changes the way the TLS value was previously meant to be
       retrieved, therefore we ensure whatever library using the old way
       gets fixed (they only exist in private tree at the moment since the
       NPTL work is still progressing).
    2) the previous number was allocated in a range causing an undefined
       instruction trap on kernels not supporting that syscall and it was
       determined that allocating it in a range returning -ENOSYS would be
       much nicer for libraries trying to determine if the feature is
       present or not.
    
    Signed-off-by: Nicolas Pitre
    Signed-off-by: default avatarRussell King <rmk+kernel@arm.linux.org.uk>
    2d2669b6
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