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2d2669b629
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: Russell King <rmk+kernel@arm.linux.org.uk> |
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