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
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