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

[H. Peter Anvin]

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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