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

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Message-ID: <4563F158.3060209@qumranet.com>
Date:	Wed, 22 Nov 2006 08:42:32 +0200
From:	Avi Kivity <avi@...ranet.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	"H. Peter Anvin" <hpa@...or.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

Jeremy Fitzhardinge wrote:
>
>
> Like "volatile" variables, I think "asm volatile" is probably overused.
> If you want to guarantee specific ordering of asms, it's probably better
> to add an explicit dependency between them rather than rely on asm
> volatile; this could either be a "memory" clobber, or something more
> fine-grained.  For example:
>
>     /* need never be instansiated; never actually referenced */
>     extern int spin_sequencer;
>
>     /* %0 never referenced */
>     asm("take spinlock" : "+m" (spin_sequencer)...);
>
>     ...
>
>     /* again, %0 never referenced */
>     asm("release spinlock" : "+m" (spin_sequencer)...);
>

Very interesting.

Will it work on load/store architectures?  Since all memory access is 
through a register, won't the constraint generate a useless register 
load (and a use of the variable)?


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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