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Date: Sun, 08 Jun 2008 00:39:43 -0500
From: Alan Cox <alc@cs.rice.edu>
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To: Kostik Belousov <kostikbel@gmail.com>
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Cc: hackers@freebsd.org
Subject: Re: Increasing KVM on amd64
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Kostik Belousov wrote:

>On Sat, Jun 07, 2008 at 06:59:35PM -0500, Alan Cox wrote:
>  
>
>>You can download a patch from 
>>http://www.cs.rice.edu/~alc/amd64_kvm_6GB.patch 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:
>`-mcmodel=kernel'
>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.

Alan