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SubjectRe: [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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