Re: [PATCH 2/5 -v2.1] x86: Detect CPUID support early at boot

From: H. Peter Anvin
Date: Sat Feb 09 2013 - 23:36:15 EST

On 02/09/2013 03:16 PM, Borislav Petkov wrote:
From: Borislav Petkov <bp@xxxxxxx>

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@xxxxxxx>
arch/x86/kernel/head_32.S | 48 +++++++++++++++++++++++------------------------
1 file changed, 23 insertions(+), 25 deletions(-)

diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
index f4d919e2cd2b..534397ba226c 100644
--- a/arch/x86/kernel/head_32.S
+++ b/arch/x86/kernel/head_32.S
@@ -318,30 +318,37 @@ 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. As a side effect, we clear DF too because GCC expects it so.
+ */
+ pushl $0
+ popfl

I wouldn't really call it a "side effect". Perhaps the right thing here is to say something like "we want to start out with %eflags unambiguously clear".

(Note also we have had to CLD earlier because we have already copied the command line.)


H. Peter Anvin, Intel Open Source Technology Center
I work for Intel. I don't speak on their behalf.

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