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x86-32: Unbreak booting on some 486 clones 

There appear to have been some 486 clones, including the "enhanced"
version of Am486, which have CPUID but not CR4.  These 486 clones had
only the FPU flag, if any, unlike the Intel 486s with CPUID, which
also had VME and therefore needed CR4.

Therefore, look at the basic CPUID flags and require at least one bit
other than bit 0 before we modify CR4.

Thanks to Christian Ludloff of sandpile.org for confirming this as a
problem.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
This commit is contained in:
H. Peter Anvin 2012-11-27 08:54:36 -08:00
parent cb7cb2864e
commit 6662c34fa9
1 changed files with 7 additions and 2 deletions
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9
arch/x86/kernel/head_32.S
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@ -292,8 +292,8 @@ default_entry:
* be using the global pages.
*
* NOTE! If we are on a 486 we may have no cr4 at all!
* Specifically, cr4 exists if and only if CPUID exists,
* which in turn exists if and only if EFLAGS.ID exists.
* Specifically, cr4 exists if and only if CPUID exists
* and has flags other than the FPU flag set.
*/
movl $X86_EFLAGS_ID,%ecx
pushl %ecx
@ -308,6 +308,11 @@ default_entry:
testl %ecx,%eax
jz 6f # No ID flag = no CPUID = no CR4
movl $1,%eax
cpuid
andl $~1,%edx # Ignore CPUID.FPU
jz 6f # No flags or only CPUID.FPU = no CR4
movl pa(mmu_cr4_features),%eax
movl %eax,%cr4