linux-kernel - Re: [PATCH 3/3] x86, AMD: Disable wrongly set X86_FEATURE_LAHF

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Message-ID: <20090830202926.GC22234@liondog.tnic>
Date:	Sun, 30 Aug 2009 22:29:26 +0200
From:	Borislav Petkov <petkovbb@...glemail.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	mingo@...hat.com, linux-kernel@...r.kernel.org,
	kjwinchester@...il.com, tglx@...utronix.de,
	borislav.petkov@....com, linux-tip-commits@...r.kernel.org
Subject: Re: [PATCH 3/3] x86, AMD: Disable wrongly set X86_FEATURE_LAHF_LM
 CPUID bit

On Sun, Aug 30, 2009 at 01:02:11PM -0700, H. Peter Anvin wrote:
> On 08/30/2009 12:30 PM, Borislav Petkov wrote:
> > On Sun, Aug 30, 2009 at 12:22:34PM -0700, H. Peter Anvin wrote:
> >> On 08/30/2009 04:50 AM, Borislav Petkov wrote:
> >>>  			clear_cpu_cap(c, X86_FEATURE_LAHF_LM);
> >>> +			if (!rdmsrl_amd_safe(0xc001100d, &val)) {
> >>> +				val &= ~(1ULL << 32);
> >>> +				wrmsr_amd_safe(0xc001100d, (u32) val,
> >>> +							   (u32)(val >> 32));
> >>> +			}
> >>> +		}
> >> We presumably want/need wrmsrl_amd_safe() here!
> > 
> > Actually, it is wrmsr_amd_safe() because we need the magic value in
> > %edi. wrmsr_amd_safe() calls the _regs variant with the array argument.
> > And we don't have a wrmsrl_amd_safe-one which gets a 64bit msr value as
> > an argument similar to the rdmsrl one.
> > 
> 
> That's exactly the point.  We shouldn't have rdmsrl_amd_safe() on one
> hand and wrmsr_asm_safe() on the other.  I have already fixed this up in
> my tree, but this kind of asymmetry should have been a big red flashing
> light.

Ok, what do we want actually? We have rdmsr_safe and rdmsrl_safe where
the last one engineers the 2 u32s into a u64. My gut feeling would opt
for the 2 32bit values instead of one 64bit since they're naturally
returned into %eax:%edx. And in the most cases we need only one of the
values. However, the MSRs themselves are 64bit... Hmmm...

-- 
Regards/Gruss,
    Boris.
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