[RFC][PATCH 1/2] x86_64: Preallocate the fixmap pud and pmd entries.

[ebiederm@xmission.com (Eric W. Biederman)]

To use the debug device I need to work through a port so I need
to call ioreamp or similar.  Since we are doing this very early
I chose to use a fixmap (because we are unlikely to free the mapping)
and because it is simple.  If we preallocate the fixmap pud and pmd
entries the existing fixmap codes works anytime from power up without
modifications or memory allocations.  So we don't need a special case.

---
 arch/x86_64/kernel/head.S |   11 ++++++++++-
 1 files changed, 10 insertions(+), 1 deletions(-)

diff --git a/arch/x86_64/kernel/head.S b/arch/x86_64/kernel/head.S
index 2f65469..4004965 100644
--- a/arch/x86_64/kernel/head.S
+++ b/arch/x86_64/kernel/head.S
@@ -271,7 +271,16 @@ NEXT_PAGE(level3_kernel_pgt)
 	.fill	510,8,0
 	/* (2^48-(2*1024*1024*1024)-((2^39)*511))/(2^30) = 510 */
 	.quad	phys_level2_kernel_pgt | 0x007
-	.fill	1,8,0
+	.quad	phys_level2_fixmap_pgt | 0x007
+
+NEXT_PAGE(level2_fixmap_pgt)
+	.fill	506,8,0
+	.quad	phys_level1_fixmap_pgt | 0x007
+	/* 8MB reserved for vsyscalls + a 2MB hole = 4 + 1 entries */
+	.fill	5,8,0
+
+NEXT_PAGE(level1_fixmap_pgt)
+	.fill	512,8,0
 
 NEXT_PAGE(level2_ident_pgt)
 	/* 40MB for bootup. 	*/
-- 
1.4.2.rc3.g7e18e-dirty