[PATCH 5/5] kexec: X86: Pass memory ranges via e820 table instead of memmap= boot parameter

H. Peter Anvin hpa at zytor.com
Thu Apr 11 11:06:50 EDT 2013

On 04/11/2013 07:55 AM, Yinghai Lu wrote:
> On Thu, Apr 11, 2013 at 5:26 AM, Thomas Renninger <trenn at suse.de> wrote:
>> Currently ranges are passed via kernel boot parameters:
>> memmap=exactmap memmap=X#Y memmap=
>> Pass them via e820 table directly instead.
> how to address "saved_max_pfn" referring in kernel?
> kernel need to use saved_max_pfn from old e820 in
> drivers/char/mem.c::read_oldmem()
> mips and powerpc they are passing that from command line "savemaxmem="
> x86 should use that too?

Oh bloody hell, yet another f-ing "max_pfn" variable.

The *only* one that makes any kind of sense is max_low_pfn (marking the
cutoff to highmem)... the pretty much the rest of them are just plain wrong.

And I don't mean "mildly annoying", I mean "catastrophically wrong
semantics".  In this case, it introduces a completely arbitrary
distinction between a nonmemory range below a high water mark and a
nonmemory range above that high water mark.  In fact, from reading the
code it seems pretty clear that the device will blindly assume that
anything below saved_max_pfn is memory and will try to map it
cachable... which will #MC on quite a few machines.

This kind of crap HAS TO STOP.  Memory is discontiguous, deal with it
and deal with it properly.

I also have to admit that I don't see the difference between /dev/mem
and /dev/oldmem, as the former allows access to memory ranges outside
the ones used by the current kernel, which is what the oldmem device
seems to be intended to od.


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

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