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SubjectRe: [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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