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    s390/ftrace: use expoline for indirect branches · caa47e1f
    Martin Schwidefsky authored
    [ Upstream commit 23a4d7fd34856da8218c4cfc23dba7a6ec0a423a ]
    
    The return from the ftrace_stub, _mcount, ftrace_caller and
    return_to_handler functions is done with "br %r14" and "br %r1".
    These are indirect branches as well and need to use execute
    trampolines for CONFIG_EXPOLINE=y.
    
    The ftrace_caller function is a special case as it returns to the
    start of a function and may only use %r0 and %r1. For a pre z10
    machine the standard execute trampoline uses a LARL + EX to do
    this, but this requires *two* registers in the range %r1..%r15.
    To get around this the 'br %r1' located in the lowcore is used,
    then the EX instruction does not need an address register.
    But the lowcore trick may only be used for pre z14 machines,
    with noexec=on the mapping for the first page may not contain
    instructions. The solution for that is an ALTERNATIVE in the
    expoline THUNK generated by 'GEN_BR_THUNK %r1' to switch to
    EXRL, this relies on the fact that a machine that supports
    noexec=on has EXRL as well.
    
    Cc: stable@vger.kernel.org # 4.16
    Fixes: f19fbd5e
    
     ("s390: introduce execute-trampolines for branches")
    Signed-off-by: default avatarMartin Schwidefsky <schwidefsky@de.ibm.com>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    caa47e1f