Increasing KVM on amd64

Alan Cox alc at
Sun Jun 8 05:39:52 UTC 2008

Kostik Belousov wrote:

>On Sat, Jun 07, 2008 at 06:59:35PM -0500, Alan Cox wrote:
>>You can download a patch from 
>> that increases amd64's 
>>kernel virtual address space to 6GB.  This patch also increases the 
>>default for the kmem map to almost 2GB.  I believe that kernel loadable 
>>modules still work.  However, I suspect that mini-dumps are broken.
>The amd64 modules text/data/bss virtual addresses are allocated from the 
>kernel_map (link_elf_obj.c, link_elf_load_file). Now, the lower end
>of the kernel_map is top-6Gb.
>Kernel code (both kernel proper and modules) is compiled for "kernel memory
>model", according to the gcc info:
>Generate code for the kernel code model.  The kernel runs in the
>negative 2 GB of the address space.  This model has to be used for
>Linux kernel code.
>I suspect we have a problem there.

The change to link_elf_obj.c is supposed to ensure allocation of an 
address in the upper 2GB of the kernel map for the module.


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