Re: [PATCH 0/6] i386 virtualization patches, Set 3

[Zachary Amsden <zach@vmware.com>]

Brian Gerst wrote:

> If you really want to test the math emu code, you can hack check_x87 
> in head.S to always leave the fpu disabled.  Then you can test it on 
> any cpu, not just a 386. 


That is a good idea, and while a valid point, it actually still requires 
writing the code to actually test the FPU, specifically, using weird 
prefixes, LDT based segments, and other oddities that don't get 
generated from "normal" C code.  I'm pretty sure the existing code works 
for the 99% cases or else it wouldn't have gotten there in the first 
place.  But testing the corner cases here is even nastier than testing 
the LDT corner cases - you would basically need to write a lot of hand 
coded i387 assembler.  Perhaps such a test might exist, but in all 
honesty, without a comprehensive test, it is simply far too easy to 
introduce a bug here, and far too likely it will either not be noticed 
until it has caused someone a possibly undetected numerical error, or 
I'm just wasting my time because noone is using this code anyways.  
Fortunately, the Hubble telescope has been upgraded to a 486 ;)

Zach