path: root/arch/arm64/include/asm/page.h
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authorArd Biesheuvel <ard.biesheuvel@linaro.org>2017-07-20 17:15:45 +0100
committerMark Rutland <mark.rutland@arm.com>2017-08-15 18:34:53 +0100
commit34be98f4944f99076f049a6806fc5f5207a755d3 (patch)
treef7d3b12acec57f6b11d2384f299fb747a482f155 /arch/arm64/include/asm/page.h
parentfork: allow arch-override of VMAP stack alignment (diff)
arm64: kernel: remove {THREAD,IRQ_STACK}_START_SP
For historical reasons, we leave the top 16 bytes of our task and IRQ stacks unused, a practice used to ensure that the SP can always be masked to find the base of the current stack (historically, where thread_info could be found). However, this is not necessary, as: * When an exception is taken from a task stack, we decrement the SP by S_FRAME_SIZE and stash the exception registers before we compare the SP against the task stack. In such cases, the SP must be at least S_FRAME_SIZE below the limit, and can be safely masked to determine whether the task stack is in use. * When transitioning to an IRQ stack, we'll place a dummy frame onto the IRQ stack before enabling asynchronous exceptions, or executing code we expect to trigger faults. Thus, if an exception is taken from the IRQ stack, the SP must be at least 16 bytes below the limit. * We no longer mask the SP to find the thread_info, which is now found via sp_el0. Note that historically, the offset was critical to ensure that cpu_switch_to() found the correct stack for new threads that hadn't yet executed ret_from_fork(). Given that, this initial offset serves no purpose, and can be removed. This brings us in-line with other architectures (e.g. x86) which do not rely on this masking. Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> [Mark: rebase, kill THREAD_START_SP, commit msg additions] Signed-off-by: Mark Rutland <mark.rutland@arm.com> Reviewed-by: Will Deacon <will.deacon@arm.com> Tested-by: Laura Abbott <labbott@redhat.com> Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: James Morse <james.morse@arm.com>
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