Re: [PATCH 3/16] read/write_crX, clts and wbinvd for 64-bit paravirt

[Jeremy Fitzhardinge <jeremy@goop.org>]

Keir Fraser wrote:
> volatile prevents the asm from being 'moved significantly', according to the
> gcc manual. I take that to mean that reordering is not allowed.
>   

That phrase doesn't appear in the gcc manual; in fact, it specifically
says that reordering can happen:

    The `volatile' keyword indicates that the instruction has important
    side-effects.  GCC will not delete a volatile `asm' if it is reachable.
    (The instruction can still be deleted if GCC can prove that
    control-flow will never reach the location of the instruction.)  Note
    that even a volatile `asm' instruction can be moved relative to other
    code, including across jump instructions.  For example, on many targets
    there is a system register which can be set to control the rounding
    mode of floating point operations.  You might try setting it with a
    volatile `asm', like this PowerPC example:

                asm volatile("mtfsf 255,%0" : : "f" (fpenv));
                sum = x + y;

    This will not work reliably, as the compiler may move the addition back
    before the volatile `asm'.  To make it work you need to add an
    artificial dependency to the `asm' referencing a variable in the code
    you don't want moved, for example:

             asm volatile ("mtfsf 255,%1" : "=X"(sum): "f"(fpenv));
             sum = x + y;

I take from this that it is not a good idea to assume "asm volatile" has
any ordering effects at all.

    J