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

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

[Zachary Amsden]

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.

Signed-off-by: Zachary Amsden <zach@vmware.com>
Index: linux-2.6.13/arch/i386/Kconfig
===================================================================
--- linux-2.6.13.orig/arch/i386/Kconfig	2005-08-24 09:30:49.000000000 -0700
+++ linux-2.6.13/arch/i386/Kconfig	2005-08-24 09:58:56.000000000 -0700
@@ -803,6 +803,14 @@ config ARCH_SELECT_MEMORY_MODEL
 	def_bool y
 	depends on ARCH_SPARSEMEM_ENABLE
 
+config MEMORY_HOLE
+	int "Create hole at top of memory (0-512 MB)"
+	range 0 512
+	default "0"
+	help
+	   Useful for creating a hole in the top of memory when running
+	   inside of a virtual machine monitor.
+
 source "mm/Kconfig"
 
 config HAVE_ARCH_EARLY_PFN_TO_NID
Index: linux-2.6.13/include/asm-i386/fixmap.h
===================================================================
--- linux-2.6.13.orig/include/asm-i386/fixmap.h	2005-08-24 09:30:43.000000000 -0700
+++ linux-2.6.13/include/asm-i386/fixmap.h	2005-08-24 10:04:42.000000000 -0700
@@ -20,7 +20,7 @@
  * Leave one empty page between vmalloc'ed areas and
  * the start of the fixmap.
  */
-#define __FIXADDR_TOP	0xfffff000
+#define __FIXADDR_TOP	(0xfffff000-(CONFIG_MEMORY_HOLE << 20))
 
 #ifndef __ASSEMBLY__
 #include <linux/kernel.h>

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

[Chris Wright]

* 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.

thanks,
-chris

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

[Zachary Amsden]

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

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

[Chris Wright]

* Zachary Amsden (zach@vmware.com) wrote:
> 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)

That's fine, I had done some braindead math anyway ;-)

>       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 ;)

Sure, good point.

thanks,
-chris