linux-kernel - Re: [PATCH]Fix broken VMI in 2.6.27-rc..

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Date:	Thu, 07 Aug 2008 17:13:33 -0700
From:	"H. Peter Anvin" <hpa@...nel.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Zachary Amsden <zach@...are.com>,
	Alok Kataria <akataria@...are.com>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	the arch/x86 maintainers <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH]Fix broken VMI in 2.6.27-rc..

Jeremy Fitzhardinge wrote:
>>
>> The Linux kernel was never a paragon of perfection - it was never 
>> meant to be.  Just because a bit of cruft went unnoticed into the 
>> kernel doesn't mean we shouldn't fix it.
> 
> I don't really see what the issue is.
> 
> Fixmaps are primarily used for things that need to be mapped early 
> before we can allocate address space dynamically.  They're predominantly 
> used for boot-time init, and rarely on any performance-critical path.  
> The only vaguely regular use a fixmap gets during runtime is poking at 
> apics, and that's dominated by IO time, and kmap_atomic.  Statically, 
> there's only 100 references in the kernel.  And it only affects 32-bit.
> 
> Having fixmaps at link-time fixed addresses would be nice, I suppose, 
> but hardly worth going to vast effort over.
> 

No, but it's hardly vast effort, either.

>>>> Given the x86 architecture, it's inevitable that PV will want to 
>>>> reserve address space at the top of memory, and therefore the fixmap 
>>>> area needs to be moved out of that space.
>>>
>>> OK.  But there's a few places where the code uses FIXADDR_TOP to mean 
>>> "top of kernel address space", so we'd need to come up with a proper 
>>> symbol for that.
>>
>> I suggest KERNEL_TOP.
> 
> Fine by me.  It would be easy to plug KERNEL_TOP/__KERNEL_TOP in now, 
> and then fix up fixmap independently.

Yes, and we should add a symbol for the bottom of the 1:1 area as well 
(to disambiguate it from TASK_SIZE).

	-hpa

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