Re: [PATCH 2/9] x86/kernel/cpu/intel.c: Initialize Enhanced REP MOVSB/STOSBenhanced

From: H. Peter Anvin
Date: Tue May 17 2011 - 23:48:05 EST

On 05/17/2011 07:46 PM, Andi Kleen wrote:
>> From: Fenghua Yu <fenghua.yu@xxxxxxxxx>
>> If kernel intends to use enhanced REP MOVSB/STOSB, it must ensure
>> IA32_MISC_ENABLE.Fast_String_Enable (bit 0) is set and CPUID.(EAX=07H,
>> ECX=0H):
>> EBX[bit 9] also reports 1.
> I suspect the check at this place is not too useful because it will
> only work for the BSP. For all others it's too late -- the patching
> has already happened.
> So either this is a problem and then it should be checked on all CPUs.
> Or maybe not at all.
> The problem is that the alternative patching currently relies on being
> run early with no other CPUs. It has no race protections, support
> for cross modification etc.
> While it would be possible to fix that it would be quite complicated
> I bet.
> So I think it's better to just remove it unless it's a real problem
> in the field.

The reason for having it is that if the BIOS has a chicken switch that
disables FAST_STRING (and it's the BIOS' responsibility to do it on all
CPUs) then this will make the kernel honor this.

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