Re: [PATCH 5/5] Create a hole in high linear address space

[Zachary Amsden <zach@vmware.com>]

Chris Wright wrote:

>* Zachary Amsden (zach@vmware.com) wrote:
>  
>
>>Allow compile time creation of a hole at the high end of linear address space.
>>This makes accomodating a hypervisor a much more tractable problem by giving
>>it ample playground to live in.  Currently, the hole size is fixed at config
>>time; I have experimented with dynamically sized holes, and have a later
>>patch that developes this potential, but it becomes much more useful once
>>the exact negotiation of linear address space with the hypervisor is defined.
>>
>>The fixed compile time solution is sufficient for now.
>>    
>>
>
>Xen moves __FIXADDR_TOP like this:
>
>#ifdef CONFIG_X86_PAE
># define HYPERVISOR_VIRT_START (0xF5800000UL)
>#else
># define HYPERVISOR_VIRT_START (0xFC000000UL)
>#endif
>
>and
>
>#define __FIXADDR_TOP  (HYPERVISOR_VIRT_START - 2 * PAGE_SIZE)
>
>and also adds bits to fixmap.
>
>So this proposed mechanism isn't quite good enough.
>  
>

Hmm.  I was thinking it would be compile time variable with defaults -- like

config MEMORY_HOLE
       int "Create hole at top of memory (0-512 MB)"
       range 0 512
       default "0"
       default 168 if (CONFIG_X86_PAE && CONFIG_X86_HYPERVISOR)
       default 64 if (!CONFIG_X86_PAE && CONFIG_X86_HYPERVISOR)
       help
          Useful for creating a hole in the top of memory when running
          inside of a virtual machine monitor.

Adding things to the fixmap is a separate concept, thus a separate patch ;)

Zach