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

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Date:	Thu, 07 Aug 2008 17:10:28 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"H. Peter Anvin" <hpa@...nel.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..

H. Peter Anvin wrote:
> Jeremy Fitzhardinge wrote:
>> H. Peter Anvin wrote:
>>> The fixmap area should never have been made movable.  It's utter 
>>> braindamage.
>>
>> Shrug.  It's been like that for a couple of years now.  It was one of 
>> the very first paravirt-ops patches.  It wasn't controversial then, 
>> and nobody seems to have noticed since.
>
> 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.

>>> 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.

    J
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