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List:       ros-dev
Subject:    Re: [ros-dev] RE: [ion] 19742:	-
From:       Alex Ionescu <ionucu () videotron ! ca>
Date:       2005-11-30 3:16:09
Message-ID: 438D1979.9060602 () videotron ! ca
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Thomas Weidenmueller wrote:

>Alex Ionescu wrote:
>  
>
>>(To be honest, my original idea was to
>>use "nop" and overwrite it with "ret", but both Patrick (who wrote the
>>patch) and MS were right in making it a "ret" by default and using "nop"
>>to overwrite, since that's even faster).
>>    
>>
>
>I don't understand how a modified nop to a ret can be faster than a ret
>generated at compile time, or vice versa. However, having a ret by
>default makes more sense ;)
>
>- Thomas
>
Simple:

If you have a "nop" by default, then your job is as follows:
- Detect support for the instruction
- If it's NOT supported, overwrite with ret.
Additionnally, the default state of the function is to WORK, which 
fundamentally speaking is wrong because you're assuming that the 
advanced functionality exists, and then you're disabling it. Since 
you're trying to be safe, then the whole point is to protect from the 
potential BSOD.
However, with a "ret" by default, you only overwrite with nop if the 
instruction IS supported. Apart from the design issue itself, I think 
the premise here is that more CPUs don't have the support for the 
function, then the ones that do.

Best regards,
Alex Ionescu

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