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

Jeremy Fitzhardinge
2007-11-01T17:41:45Z

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