Messages in this thread |  | | Date | Sat, 09 Feb 2013 20:34:53 -0800 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH 2/5 -v2.1] x86: Detect CPUID support early at boot |
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On 02/09/2013 03:16 PM, Borislav Petkov wrote: > 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, 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.)
-hpa
-- H. Peter Anvin, Intel Open Source Technology Center I work for Intel. I don't speak on their behalf.
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