Re: [PATCH 04/13] x86, mm: Revert back good_end setting for 64bit

[H. Peter Anvin]

On 10/05/2012 05:28 PM, Eric W. Biederman wrote:
> > Seriously, any case where we can't load anywhere in physical ram on x86-64 is a
> > bug.  i386 is another matter.
>
> As I recall there are data structures like the IDT that only have a
> 32bit base address.

Not true.  The only one I know of is memory for the trampoline which has 
to be below 1M.  The < 1M space is already handled specially for good 
reason.

> According to the bzImage header we don't support ramdisks above 4G.
> I think we also have a 32bit address for the kernel command line
> in the bzImage header.

There are pointers in the bzImage header, that is true.  We can fix that 
problem, though, at least for entry via the 64-bit entry point.

> In the case of kdump in particular there is a need for DMAable
> memory and in general that means memory below 4G.  So as long
> as we only support one memory extent for kdump it makes sense
> for that segment to be below 4G.

"In general" meaning "no iotlb"?  In that case you have some unknown 
address space restriction which may or may not be 4G...

> For a normal x86_64 kernel which gets to use most of the memory it
> definitely should be loadable anywhere in memory.

Yes.  We should fix problems, like the limitations in the boot_params 
structure.

	-hpa

--
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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