linux-kernel - Re: [kvm-devel] [PATCH] KVM: Avoid using vmx instruction directly

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Message-ID: <4554D538.10404@goop.org>
Date:	Fri, 10 Nov 2006 11:38:32 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Martin Schwidefsky <schwidefsky@...glemail.com>
CC:	Avi Kivity <avi@...ranet.com>, Arnd Bergmann <arnd@...db.de>,
	kvm-devel@...ts.sourceforge.net, akpm@...l.org,
	linux-kernel@...r.kernel.org
Subject: Re: [kvm-devel] [PATCH] KVM: Avoid using vmx instruction directly

Martin Schwidefsky wrote:
> On 11/10/06, Jeremy Fitzhardinge <jeremy@...p.org> wrote:
>> >> Or gcc
>> >> might move the assignment of phys_addr to after the inline assembly.
>> >>
>> > "asm volatile" prevents that (and I'm not 100% sure it's necessary).
>>
>> No, it won't necessarily.  "asm volatile" simply forces gcc to emit the
>> assembler, even if it thinks its output doesn't get used.  It makes no
>> ordering guarantees with respect to other code (or even other "asm
>> volatiles").   The "memory" clobbers should fix the ordering of the asms
>> though.
>
> The "memory" clobber just tells the compiler that any memory object
> might get access by the inline. 

I just meant that two asms with a "memory" clobber will be generated
with a fixed ordering, which "asm volatile" does not necessarily do.

    J
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