• David S. Miller's avatar
    sparc64: Make montmul/montsqr/mpmul usable in 32-bit threads. · 517ffce4
    David S. Miller authored
    The Montgomery Multiply, Montgomery Square, and Multiple-Precision
    Multiply instructions work by loading a combination of the floating
    point and multiple register windows worth of integer registers
    with the inputs.
    
    These values are 64-bit.  But for 32-bit userland processes we only
    save the low 32-bits of each integer register during a register spill.
    This is because the register window save area is in the user stack and
    has a fixed layout.
    
    Therefore, the only way to use these instruction in 32-bit mode is to
    perform the following sequence:
    
    1) Load the top-32bits of a choosen integer register with a sentinel,
       say "-1".  This will be in the outer-most register window.
    
       The idea is that we're trying to see if the outer-most register
       window gets spilled, and thus the 64-bit values were truncated.
    
    2) Load all the inputs for the montmul/montsqr/mpmul instruction,
       down to the inner-most register window.
    
    3) Execute the opcode.
    
    4) Traverse back up to the outer-most register window.
    
    5) Check the sentinel, if it's still "-1" store the results.
       Otherwise retry the entire sequence.
    
    This retry is extremely troublesome.  If you're just unlucky and an
    interrupt or other trap happens, it'll push that outer-most window to
    the stack and clear the sentinel when we restore it.
    
    We could retry forever and never make forward progress if interrupts
    arrive at a fast enough rate (consider perf events as one example).
    So we have do limited retries and fallback to software which is
    extremely non-deterministic.
    
    Luckily it's very straightforward to provide a mechanism to let
    32-bit applications use a 64-bit stack.  Stacks in 64-bit mode are
    biased by 2047 bytes, which means that the lowest bit is set in the
    actual %sp register value.
    
    So if we see bit zero set in a 32-bit application's stack we treat
    it like a 64-bit stack.
    
    Runtime detection of such a facility is tricky, and cumbersome at
    best.  For example, just trying to use a biased stack and seeing if it
    works is hard to recover from (the signal handler will need to use an
    alt stack, plus something along the lines of longjmp).  Therefore, we
    add a system call to report a bitmask of arch specific features like
    this in a cheap and less hairy way.
    
    With help from Andy Polyakov.
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    517ffce4
thread_info_64.h 8.66 KB