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

(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.
>> -------------------------------------------------------

Anyone have a solution to this? :)

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