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Subject[PATCH 2/5] x86: Detect CPUID support early at boot
Date
From: Borislav Petkov <bp@suse.de>

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>
---
arch/x86/kernel/head_32.S | 48 +++++++++++++++++++++++------------------------
1 file changed, 24 insertions(+), 24 deletions(-)

diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
index f4d919e2cd2b..df0b324d2854 100644
--- a/arch/x86/kernel/head_32.S
+++ b/arch/x86/kernel/head_32.S
@@ -318,30 +318,38 @@ default_entry:
movl %eax,%cr0

/*
- * New page tables may be in 4Mbyte page mode and may
- * be using the global pages.
+ * Initialize EFLAGS. Some BIOSes leave bits like NT set. This would confuse the
+ * debugger if this code is traced. Best to initialize before switching to
+ * protected mode.
+ */
+
+ 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
@@ -389,14 +397,6 @@ 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.
*/
--
1.8.1.3.535.ga923c31


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