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Re: [Qemu-devel] [Bug 645662] x87 fpu emulation not accurate enough

From: KONRAD Frederic
Subject: Re: [Qemu-devel] [Bug 645662] x87 fpu emulation not accurate enough
Date: Wed, 29 Nov 2017 11:17:18 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1

I would write some more basic tests. eg: at instruction level and
compare KVM vs TCG mode to find out which trigo instructions are
differents (probably not only one).

BTW I'm not floating point expert at all :).


On 11/29/2017 10:59 AM, william lin wrote:

The goal of the project is to fix the bug. Sorry I should have been more

This bug has been around for 7+ years now. Qemu switched to softfloat and
it did not fix this. I think its most likely caused by Qemu's internal
representation not being accurate enough or accuracy lost during
conversion. However this is basically what is given to us in the bug
report, and I have not been able to make much progress past this point, so
we are asking for help.


On Nov 29, 2017 3:39 AM, "KONRAD Frederic" <address@hidden>


I have the same problem here with some of our math testsuites.
It's about some precision issue with trigonometric tests.

I didn't had time to figure out what happen neither what is
really going wrong. eg: trigonometric function or?

A quick workaround is to use KVM when possible.


On 11/29/2017 09:34 AM, william lin wrote:


I have been looking into this bug
<https://bugs.launchpad.net/qemu/+bug/645662> for a school project, and

after reproducing the bug and tracing through the code, I have run into a
wall trying to figure out the best way to approach this. If there is anyone
who is familiar with qemu's fpu and is willing, any tips or suggestions
will be fantastic.


Will & Wes

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