x86: Detect CPUID support early at boot

We detect CPUID function support on each CPU and save it for later use,
obviating the need to play the toggle EFLAGS.ID game every time. C code
is looking at ->cpuid_level anyway.

Signed-off-by: Borislav Petkov <bp@suse.de>
Link: http://lkml.kernel.org/r/1360592538-10643-3-git-send-email-bp@alien8.de
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

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
@@ -378,14 +387,6 @@ default_entry:
        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)