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

[Jeremy Fitzhardinge]

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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