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    arm64: kernel: remove {THREAD,IRQ_STACK}_START_SP · 34be98f4
    Ard Biesheuvel authored
    
    
    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: default avatarArd Biesheuvel <ard.biesheuvel@linaro.org>
    [Mark: rebase, kill THREAD_START_SP, commit msg additions]
    Signed-off-by: default avatarMark Rutland <mark.rutland@arm.com>
    Reviewed-by: default avatarWill Deacon <will.deacon@arm.com>
    Tested-by: default avatarLaura Abbott <labbott@redhat.com>
    Cc: Catalin Marinas <catalin.marinas@arm.com>
    Cc: James Morse <james.morse@arm.com>
    34be98f4