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    SubjectRe: [PATCH 1/11] Add generic helpers for arch IPI function calls
    Date
    James Bottomley <James.Bottomley@HansenPartnership.com> wrote:

    > Erm, that's a bug in the frv toolchain, isn't it? The linker should
    > *never* rely on C code annotation for jump lengths ... mainly because
    > you can screw it all up again in the linker script, so the sectional
    > annotations should only be in the function body (think
    > -ffunction-sections)

    The problem with FRV has to do with global variables, not functions.

    With FRV, we attempt to pack as many small variables into the "small data
    section" (.sdata) as we can, and then retain a pointer to it in a register.
    Thus for small variables, we can use a single instruction, such as:

    LDI @(GR16,%gprel(my_variable)),GRx

    to access it. However, this limits the offset of my_variable to be +/-2048
    from GR16 because all the instructions are the same size, and that's the size
    of the offset field therein.

    Alternatively, we can use:

    SETHI.P %hi(my_variable),GRy
    SETLO %lo(my_variable),GRy
    LD @(GRy,GR0),GRx

    which obviously takes two more instructions, although they can be executed
    simultaneously, and 8 more bytes of RAM/icache.

    However, the compiler will assume that it should access _all_ global variables
    using GPREL-addressing unless it is told otherwise. This means that if a
    global variable should not be GPREL-addressed, its *declaration* should be
    annotated, as this affects how it is addressed. Furthermore, if the variable
    is *defined* in C, that definition should be annotated the same as the
    declaration, otherwise it'll eat storage from .sdata rather than the
    appropriate section.


    Global functions are another matter. The FRV's PC+offset call and branch
    instructions have 24-bit displacements, which is fine for leaping about within
    the kernel core with a single instruction.

    In modules, we instead (a) suppress GPREL addressing entirely because we can't
    put module variables in .sdata, and (b) make all interfunction jumps that the
    assembler can't resolve into:

    SETHI.P %hi(that_function),GRy
    SETLO %lo(that_function),GRy
    JMPL @(GRy,GR0)

    because a 24-bit offset can only span 16MB, which isn't far enough.


    We could use a GOT, but I believe that would add an extra memory access to any
    memory access or jump - which I'd prefer to avoid.

    David


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