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Re: [Xen-devel] 64 bit Intel HVM Weirdness - setjmp\longjump fp exceptio

To: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] 64 bit Intel HVM Weirdness - setjmp\longjump fp exception in Xen, not on real hardware or AMD HVM
From: Allan Graves <allan.graves@xxxxxxxxxx>
Date: Wed, 04 Nov 2009 13:54:17 -0500
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Thank you. Umm... I guess I wasn't sure exactly what the changelog should say - but clearly something other than what I would have titled it. Which is probably cause I don't think I could have fixed the bug. :)

In any case, 3.4.1 does fix the issue!

Thank you so much for helping me through this!


Keir Fraser wrote:
Well, what would you expect the changelog to say? It's fixed in the
xen-3.4-testing.hg repository by changeset 19697, which is a backport of

 -- Keir

On 01/11/2009 19:32, "Allan Graves" <allan.graves@xxxxxxxxxx> wrote:

Keir -

Hmm... according to /sys/hypervisor/version, I'm running 3.4.0.

Is there a reasonable chance this is fixed in 3.4.1?  I haven't seen
anything in the changelogs...


Keir Fraser wrote:
What version of Xen are you using? This bug may be fixed in Xen 3.3.2 and
Xen 3.4.1.

 -- Keir

On 30/10/2009 01:23, "Allan Graves" <allan.graves@xxxxxxxxxx> wrote:

(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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