Linux Virtualization - Apr 2007 - [PATCH 0/12] Early USB debug port and i386 boot cleanups

Modern hardware relies primarily on memory mapped I/O which is typically
at addresses that are not mapped by the kernels initial page tables,
which makes using them currently unusable for early debugging print support.

So this patch set digs in and fixes the early page tables on both
arch/i386 and arch/x86_64 so that set_fixmap works with our initial boot
page tables.   All that is needed is that we allocate preallocate the pte
page that the fixmap entries live on.

On arch/i386 I accomplish this by ensuring the boot time page tables are
in the native mode.  Doing this provides an alternative and slightly
more capable fix to Jeremy's problem of not having the initial memory
mapping covering enough pages to identity map all of low memory.  Because
we are now updating the page table we are running on we add the mappings
we need for the initial page table as we build the initial page table.

Since I was in there I kept cleaning up arch/i386/head.S.
In particular I have gotten as far as removing the early cpuid,
and have added a head32.S.

Eric

This patch preallocates the intermediate page table entries so that
all that is needed to setup a fixmap is to fill in the appropriate
pte.

By doing this modern hardware that uses memory mapped access can be
talked to early in boot through a fixmap. 

Allowing USB debugging and the like.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
---
 arch/x86_64/kernel/head.S |   14 +++++++++++++-
 1 files changed, 13 insertions(+), 1 deletions(-)

diff --git a/arch/x86_64/kernel/head.S b/arch/x86_64/kernel/head.S
index 1fab487..1b704c4 100644
--- a/arch/x86_64/kernel/head.S
+++ b/arch/x86_64/kernel/head.S
@@ -74,6 +74,9 @@ startup_64:
 
 	addq	%rbp, level3_ident_pgt + 0(%rip)
 	addq	%rbp, level3_kernel_pgt + (510*8)(%rip)
+	addq	%rbp, level3_kernel_pgt + (511*8)(%rip)
+
+	addq	%rbp, level2_fixmap_pgt + (506*8)(%rip)
 
 	/* Add an Identity mapping if I am above 1G */
 	leaq	_text(%rip), %rdi
@@ -314,7 +317,16 @@ NEXT_PAGE(level3_kernel_pgt)
 	.fill	510,8,0
 	/* (2^48-(2*1024*1024*1024)-((2^39)*511))/(2^30) = 510 */
 	.quad	level2_kernel_pgt - __START_KERNEL_map + _KERNPG_TABLE
-	.fill	1,8,0
+	.quad	level2_fixmap_pgt - __START_KERNEL_map + _KERNPG_TABLE + _PAGE_USER
+
+NEXT_PAGE(level2_fixmap_pgt)
+	.fill	506,8,0
+	.quad	level1_fixmap_pgt - __START_KERNEL_map + _KERNPG_TABLE + _PAGE_USER
+	/* 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)
 	/* Since I easily can, map the first 1G.
-- 
1.5.1.1.181.g2de0

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