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authorNicolas Pitre <nico@org.rmk.(none)>2005-04-29 17:08:33 -0400
committerRussell King <rmk+kernel@arm.linux.org.uk>2005-04-29 17:08:33 -0400
commit2d2669b62984b8d76b05a6a045390a3250317d21 (patch)
tree822f62adf59f2e6302a16289cc99b0f9b873cfb4 /arch/arm/mm/Kconfig
parent3a1e501511a1e2c665c566939047794dcf86466b (diff)
[PATCH] ARM: 2651/3: kernel helpers for NPTL support
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>
Diffstat (limited to 'arch/arm/mm/Kconfig')
-rw-r--r--arch/arm/mm/Kconfig14
1 files changed, 14 insertions, 0 deletions
diff --git a/arch/arm/mm/Kconfig b/arch/arm/mm/Kconfig
index 5b670c9ac5ef..007766a0644c 100644
--- a/arch/arm/mm/Kconfig
+++ b/arch/arm/mm/Kconfig
@@ -409,3 +409,17 @@ config CPU_BPREDICT_DISABLE
409 depends on CPU_ARM1020 409 depends on CPU_ARM1020
410 help 410 help
411 Say Y here to disable branch prediction. If unsure, say N. 411 Say Y here to disable branch prediction. If unsure, say N.
412
413config HAS_TLS_REG
414 bool
415 depends on CPU_32v6 && !CPU_32v5 && !CPU_32v4 && !CPU_32v3
416 help
417 This selects support for the CP15 thread register.
418 It is defined to be available on ARMv6 or later. However
419 if the kernel is configured to support multiple CPUs including
420 a pre-ARMv6 processors, or if a given ARMv6 processor doesn't
421 implement the thread register for some reason, then access to
422 this register from user space must be trapped and emulated.
423 If user space is relying on the __kuser_get_tls code then
424 there should not be any impact.
425