Linux Virtualization - Apr 2007 - [RFC, PATCH 7/24] i386 Vmi memory hole

Create a configurable hole in the linear address space at the top
of memory.  A more advanced interface is needed to negotiate how
much space the hypervisor is allowed to steal, but in the end, it
seems most likely that a fixed constant size will be chosen for
the compiled kernel, potentially propagated to an information
page used by paravirtual initialization to determine interface
compatibility.

Signed-off-by: Zachary Amsden <zach@vmware.com>

Index: linux-2.6.16-rc3/arch/i386/Kconfig
==================================================================---
linux-2.6.16-rc3.orig/arch/i386/Kconfig	2006-02-22 16:09:04.000000000 -0800
+++ linux-2.6.16-rc3/arch/i386/Kconfig	2006-02-22 16:33:27.000000000 -0800
@@ -201,6 +201,15 @@ config VMI_DEBUG
 
 endmenu
 
+config MEMORY_HOLE
+	int "Create hole at top of memory (0-256 MB)"
+	range 0 256
+	default "64" if X86_VMI
+	default "0" if !X86_VMI
+	help
+	   Useful for creating a hole in the top of memory when running
+	   inside of a virtual machine monitor.
+
 config ACPI_SRAT
 	bool
 	default y
Index: linux-2.6.16-rc3/include/asm-i386/fixmap.h
==================================================================---
linux-2.6.16-rc3.orig/include/asm-i386/fixmap.h	2006-02-22 15:48:23.000000000
-0800
+++ linux-2.6.16-rc3/include/asm-i386/fixmap.h	2006-02-22 16:33:27.000000000
-0800
@@ -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>

* Zachary Amsden (zach@vmware.com) wrote:
> Create a configurable hole in the linear address space at the top
> of memory.  A more advanced interface is needed to negotiate how
> much space the hypervisor is allowed to steal, but in the end, it
> seems most likely that a fixed constant size will be chosen for
> the compiled kernel, potentially propagated to an information
> page used by paravirtual initialization to determine interface
> compatibility.
> 
> Signed-off-by: Zachary Amsden <zach@vmware.com>
> 
> Index: linux-2.6.16-rc3/arch/i386/Kconfig
> ==================================================================> ---
linux-2.6.16-rc3.orig/arch/i386/Kconfig	2006-02-22 16:09:04.000000000 -0800
> +++ linux-2.6.16-rc3/arch/i386/Kconfig	2006-02-22 16:33:27.000000000 -0800
> @@ -201,6 +201,15 @@ config VMI_DEBUG
>  
>  endmenu
>  
> +config MEMORY_HOLE
> +	int "Create hole at top of memory (0-256 MB)"
> +	range 0 256
> +	default "64" if X86_VMI
> +	default "0" if !X86_VMI
Deja-vu ;-)  And still works in context of Xen, but we've just let the
subarch define the __FIXADDR_TOP.  Having it be dynamic could be
interesting.