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

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Message-ID: <472A0FBF.6040907@goop.org>
Date:	Thu, 01 Nov 2007 10:41:19 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Keir Fraser <Keir.Fraser@...cam.ac.uk>
CC:	Glauber de Oliveira Costa <gcosta@...hat.com>, lguest@...abs.org,
	kvm-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
	ak@...e.de, chrisw@...s-sol.org, tglx@...utronix.de,
	anthony@...emonkey.ws, akpm@...ux-foundation.org,
	virtualization@...ts.linux-foundation.org, mingo@...e.hu
Subject: Re: [PATCH 3/16] read/write_crX, clts and wbinvd for 64-bit paravirt

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
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