Xen users - Oct 2009 - 64 bit Intel HVM Weirdness - setjmp\longjump fp exception in Xen, not on real hardware or AMD HVM

The following program will crash with a fp exception on an INTEL 64 bit HVM domU
only:
#include <setjmp.h>
jmp_buf env; main() { if(setjmp(env)) return; longjmp(env, 1); }


This can be seen using both Linux and Windows 64 bit Guests.


My machine is a Core 2 Duo with both VT-D and VT-X extensions enabled.  

Unfortunately, this is blocking our usage of 64 bit HVM Windows guests.  

A similar issue was reported in KVM and recently fixed, some of their analysis
is below:
--------------------------------------------------
> >> It seems that the problem can be reproduced by compiling the
> >> following simple program using cygwin''s gcc. The program
crashes on
> >> w2k3-amd64 on kvm-83 on core2-duo, and it does not crash on the
> >> same w2k3-amd64 installation on kvm-83 on AMD Phenom.
> >>
> >> #include <setjmp.h>
> >>
> >> jmp_buf env; main() { if(setjmp(env)) return; longjmp(env, 1); }
> >>
> >> The problem seems to be in the instruction ``mov
gs,ax'''' (Intel
> >> syntax) in the longjmp() code. If I let the virtual machine
execute
> >> the instruction, the program crashes. However, if I step over the 
> >> instruction using the vs2008 debugger, the program completes
> >> without crashing. Thus, I think that this is the instruction that
> >> Avi is looking for, but I don''t know how to proceed from
here.
> >> -------------------------------------------------------
http://markmail.org/message/owy3x7pf6oywdx5e#query:+page:1+mid:spz6vmdothb2njfb+state:results


Anyone have a solution to this? :)  


Allan




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On Thu, Oct 29, 2009 at 07:18:46PM -0400, Allan Graves
wrote:
> The following program will crash with a fp exception on an INTEL 64 bit HVM
> domU only:
> #include <setjmp.h>
> jmp_buf env; main() { if(setjmp(env)) return; longjmp(env, 1); }
> 
> 
> This can be seen using both Linux and Windows 64 bit Guests.
> 
> 
> My machine is a Core 2 Duo with both VT-D and VT-X extensions enabled.  
> 
> Unfortunately, this is blocking our usage of 64 bit HVM Windows guests.  
> 
> A similar issue was reported in KVM and recently fixed, some of their 
> analysis is below:
> --------------------------------------------------
> >>> It seems that the problem can be reproduced by compiling the
> >>> following simple program using cygwin''s gcc. The
program crashes on
> >>> w2k3-amd64 on kvm-83 on core2-duo, and it does not crash on
the
> >>> same w2k3-amd64 installation on kvm-83 on AMD Phenom.
> >>>
> >>> #include <setjmp.h>
> >>>
> >>> jmp_buf env; main() { if(setjmp(env)) return; longjmp(env, 1);
}
> >>>
> >>> The problem seems to be in the instruction ``mov
gs,ax'''' (Intel
> >>> syntax) in the longjmp() code. If I let the virtual machine
execute
> >>> the instruction, the program crashes. However, if I step over
the
> >>> instruction using the vs2008 debugger, the program completes
> >>> without crashing. Thus, I think that this is the instruction
that
> >>> Avi is looking for, but I don''t know how to proceed
from here.
> >>> -------------------------------------------------------
>
http://markmail.org/message/owy3x7pf6oywdx5e#query:+page:1+mid:spz6vmdothb2njfb+state:results
> 
> 
> Anyone have a solution to this? :)  
>
I think this should be sent to xen-devel instead :)

-- Pasi


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