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Re: [Xen-devel] Xen guest floating point problem

To: "Mick.Jordan@xxxxxxx" <Mick.Jordan@xxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Xen guest floating point problem
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Tue, 04 Aug 2009 09:16:05 +0100
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On 04/08/2009 00:51, "Mick Jordan" <Mick.Jordan@xxxxxxx> wrote:

> We have a simple C test program that runs on our evolved version of
> Mini-OS (GUK) that exhibits sporadic non-deterministic failure regarding
> floating point computations (mostly calling the __ieee754_remainder
> method of fdlibm). Typically about 5 computations in 10000 fail, usually
> with Nan or Infinite values showing up. We are as convinced as we can be
> at this point that the GUK code is not corrupting the floating point
> state, so are down to two ideas. One is a bug in Xen on guest context
> switching or event delivery regarding the FP state. The other is that
> GUK is failing to obey some Xen API contract regarding FP state. For
> example, timer and xenstore events are being delivered during the run
> although we can't correlate them directly to the fault.
> The program fails on both Solaris xVM 3.1.4, xVM 3.3, various Linux
> 3.1.4 systems, but does not fail when run on Solaris or Linux domUs.
> This points the finger at GUK but we are wondering whether they have a
> workaround for some problem we are not aware of.

I don't think the guest should have to do anything special regarding
handling FP state. Xen should automatically save/restore the state as
necessary. We had bugs in this area long ago, but they were fixed and I'd be
surprised if problems had crept back in.

 -- Keir

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