x86: Detect CPUID support early at boot

[pandora-kernel.git] / arch / x86 / kernel / head_32.S

diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
index a9c5cc8..e3725a0 100644 (file)
--- a/arch/x86/kernel/head_32.S
+++ b/arch/x86/kernel/head_32.S
@@ -307,30 +307,39 @@ default_entry:
        movl %eax,%cr0
 
 /*
- *     New page tables may be in 4Mbyte page mode and may
- *     be using the global pages. 
+ * We want to start out with EFLAGS unambiguously cleared. Some BIOSes leave
+ * bits like NT set. This would confuse the debugger if this code is traced. So
+ * initialize them properly now before switching to protected mode. That means
+ * DF in particular (even though we have cleared it earlier after copying the
+ * command line) because GCC expects it.
+ */
+       pushl $0
+       popfl
+
+/*
+ * 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!
- *     Specifically, cr4 exists if and only if CPUID exists
- *     and has flags other than the FPU flag set.
+ * NOTE! If we are on a 486 we may have no cr4 at all! Specifically, cr4 exists
+ * if and only if CPUID exists and has flags other than the FPU flag set.
  */
+       movl $-1,pa(X86_CPUID)          # preset CPUID level
        movl $X86_EFLAGS_ID,%ecx
        pushl %ecx
-       popfl
-       pushfl
-       popl %eax
-       pushl $0
-       popfl
+       popfl                           # set EFLAGS=ID
        pushfl
-       popl %edx
-       xorl %edx,%eax
-       testl %ecx,%eax
-       jz 6f                   # No ID flag = no CPUID = no CR4
+       popl %eax                       # get EFLAGS
+       testl $X86_EFLAGS_ID,%eax       # did EFLAGS.ID remained set?
+       jz 6f                           # hw disallowed setting of ID bit
+                                       # which means no CPUID and no CR4
+
+       xorl %eax,%eax
+       cpuid
+       movl %eax,pa(X86_CPUID)         # save largest std CPUID function
 
        movl $1,%eax
        cpuid
-       andl $~1,%edx           # Ignore CPUID.FPU
-       jz 6f                   # No flags or only CPUID.FPU = no CR4
+       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
@@ -377,14 +386,6 @@ default_entry:
        /* Shift the stack pointer to a virtual address */
        addl $__PAGE_OFFSET, %esp
 
-/*
- * Initialize eflags.  Some BIOS's leave bits like NT set.  This would
- * confuse the debugger if this code is traced.
- * XXX - best to initialize before switching to protected mode.
- */
-       pushl $0
-       popfl
-
 /*
  * start system 32-bit setup. We need to re-do some of the things done
  * in 16-bit mode for the "real" operations.
@@ -461,7 +462,6 @@ is486:      movl $0x50022,%ecx      # set AM, WP, NE and MP
        xorl %eax,%eax                  # Clear LDT
        lldt %ax
 
-       cld                     # gcc2 wants the direction flag cleared at all times
        pushl $0                # fake return address for unwinder
        jmp *(initial_code)