linux-kernel - Re: [PATCH] i386: Decouple PAE from CONFIG

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Message-ID: <469EBC63.9020803@zytor.com>
Date:	Wed, 18 Jul 2007 18:20:35 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Avi Kivity <avi@...ranet.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	kvm-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
	caglar@...dus.org.tr
Subject: Re: [PATCH] i386: Decouple PAE from CONFIG_CMPXCHG64

Andi Kleen wrote:
>>
>> No processors that support KVM exist that also do not support CMPXCHG64,
>> so no additional check is necessary.  This setup allows for a single kernel
>> that will boot on i486 and also support KVM if available.
> 
> The CONFIG should only control the early CPUID checks, which are needed for PAE
> kernels.
> 
> But for something late like KVM the correct fix is really to drop
> the ifdefs around the macro and check boot_cpu_has() yourself.
> 

He's checking for the appropriate functionality at runtime.  What this
does is makes it impossible to select KVM when you have already selected
a CPU that doesn't support CX8.

One can, of course, question the utility, since the CPU set that
supports KVM all support CX8, but not vice versa...

	-hpa
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