[Xen-devel] 64 bit Intel HVM Weirdness - setjmp\longjump fp exception in Xen, not on real hardware or AMD HVM

[Allan Graves <allan.graves@xxxxxxxxxx>]

(Please excuse double posting - I was told xen-users was not the right 
list, to put it on xen-devel, as it was not a technical support query, 
but an issue.)
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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