linux-kernel - Re: [GIT PULL][RESEND #2] KVM Updates for 2.6.23-rc1

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Message-ID: <alpine.LFD.0.999.0707171212070.27353@woody.linux-foundation.org>
Date:	Tue, 17 Jul 2007 12:15:06 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"H. Peter Anvin" <hpa@...or.com>
cc:	caglar@...dus.org.tr, Avi Kivity <avi@...ranet.com>,
	linux-kernel@...r.kernel.org, kvm-devel@...ts.sourceforge.net,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [GIT PULL][RESEND #2] KVM Updates for 2.6.23-rc1



On Tue, 17 Jul 2007, H. Peter Anvin wrote:
>
> S.Çağlar Onur wrote:
> > 
> > If i'm not wrong X86_CMPXCHG64 depends on CONFIG_X86_PAE which depends on 
> > HIGHMEM64 and again if i'm not wrong this means distributions who wants to 
> > provide KVM must enable CONFIG_X86_PAE and CONFIG_HIGHMEM64G from now on?
> > 
> 
> X86_PAE should depend on X86_CMPXCHG64, not the other way around.

Actually, I think the *real* solution would be:

 - add a X86_HAS_CMPXCHG8B config option, and set it for the appropriate 
   CPU selection (P6 and up, or whatever the rule is)

 - make KVM depend on it

 - make KVM and HIGHMEM64 _select_ another config option, namely the 
   NEEDS_CMPXCHG8B

and then we make the cpufeatures code check the CMPXCHG bit only if the 
NEEDS_CMPXCHG8B thing is set. That gives us the best of all worlds.

Because there is no point in checking whether the CPU supports it if the 
kernel doesn't _need_ it. Especially since we know that some CPU's lie 
about it due to old NT bugs.

		Linus
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