x86-32: Start out eflags and cr4 clean

%cr4 is supposed to reflect a set of features into which the operating
system is opting in.  If the BIOS or bootloader leaks bits here, this
is not desirable.  Consider a bootloader passing in %cr4.pae set to a
legacy paging kernel, for example -- it will not have any immediate
effect, but the kernel would crash when turning paging on.

A similar argument applies to %eflags, and since we have to look for
%eflags.id being settable we can use a sequence which clears %eflags
as a side effect.

Note that we already do this for x86-64.
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Link: http://lkml.kernel.org/r/1348529239-17943-1-git-send-email-hpa@linux.intel.com

arch/x86/kernel/head_32.S

@@ -287,27 +287,28 @@ ENTRY(startup_32_smp)
 	leal -__PAGE_OFFSET(%ecx),%esp
 
 default_entry:
-
 /*
  *	New page tables may be in 4Mbyte page mode and may
  *	be using the global pages. 
  *
  *	NOTE! If we are on a 486 we may have no cr4 at all!
- *	So we do not try to touch it unless we really have
- *	some bits in it to set.  This won't work if the BSP
- *	implements cr4 but this AP does not -- very unlikely
- *	but be warned!  The same applies to the pse feature
- *	if not equally supported. --macro
- *
- *	NOTE! We have to correct for the fact that we're
- *	not yet offset PAGE_OFFSET..
+ *	Specifically, cr4 exists if and only if CPUID exists,
+ *	which in turn exists if and only if EFLAGS.ID exists.
  */
-#define cr4_bits pa(mmu_cr4_features)
-	movl cr4_bits,%edx
-	andl %edx,%edx
-	jz 6f
-	movl %cr4,%eax		# Turn on paging options (PSE,PAE,..)
-	orl %edx,%eax
+	movl $X86_EFLAGS_ID,%ecx
+	pushl %ecx
+	popfl
+	pushfl
+	popl %eax
+	pushl $0
+	popfl
+	pushfl
+	popl %edx
+	xorl %edx,%eax
+	testl %ecx,%eax
+	jz 6f			# No ID flag = no CPUID = no CR4
+
+	movl pa(mmu_cr4_features),%eax
 	movl %eax,%cr4
 
 	testb $X86_CR4_PAE, %al		# check if PAE is enabled