x86-32: Start out cr0 clean, disable paging before modifying cr3/4

Patch

  5a5a51db78e x86-32: Start out eflags and cr4 clean

... made x86-32 match x86-64 in that we initialize %eflags and %cr4
from scratch.  This broke OLPC XO-1.5, because the XO enters the
kernel with paging enabled, which the kernel doesn't expect.

Since we no longer support 386 (the source of most of the variability
in %cr0 configuration), we can simply match further x86-64 and
initialize %cr0 to a fixed value -- the one variable part remaining in
%cr0 is for FPU control, but all that is handled later on in
initialization; in particular, configuring %cr0 as if the FPU is
present until proven otherwise is correct and necessary for the probe
to work.

To deal with the XO case sanely, explicitly disable paging in %cr0
before we muck with %cr3, %cr4 or EFER -- those operations are
inherently unsafe with paging enabled.

NOTE: There is still a lot of 386-related junk in head_32.S which we
can and should get rid of, however, this is intended as a minimal fix
whereas the cleanup can be deferred to the next merge window.

Reported-by: Andres Salomon <dilinger@queued.net>
Tested-by: Daniel Drake <dsd@laptop.org>
Link: http://lkml.kernel.org/r/50FA0661.2060400@linux.intel.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/kernel/head_32.S

@@ -300,6 +300,12 @@ ENTRY(startup_32_smp)
 	leal -__PAGE_OFFSET(%ecx),%esp
 
 default_entry:
+#define CR0_STATE	(X86_CR0_PE | X86_CR0_MP | X86_CR0_ET | \
+			 X86_CR0_NE | X86_CR0_WP | X86_CR0_AM | \
+			 X86_CR0_PG)
+	movl $(CR0_STATE & ~X86_CR0_PG),%eax
+	movl %eax,%cr0
+
 /*
  *	New page tables may be in 4Mbyte page mode and may
  *	be using the global pages. 
@@ -364,8 +370,7 @@ default_entry:
  */
 	movl $pa(initial_page_table), %eax
 	movl %eax,%cr3		/* set the page table pointer.. */
-	movl %cr0,%eax
-	orl  $X86_CR0_PG,%eax
+	movl $CR0_STATE,%eax
 	movl %eax,%cr0		/* ..and set paging (PG) bit */
 	ljmp $__BOOT_CS,$1f	/* Clear prefetch and normalize %eip */
 1: