linux-kernel - Floating point problems on UML

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Message-ID: <4E4C2B6F.7000800@nod.at>
Date:	Wed, 17 Aug 2011 22:58:23 +0200
From:	Richard Weinberger <richard@....at>
To:	linux-kernel@...r.kernel.org
CC:	user-mode-linux-devel@...ts.sourceforge.net,
	gunnarlindroth@...mail.com
Subject: Floating point problems on UML - help needed

Hi,

Gunnar reported that some Java program does not work proper within 
UserModeLinux.
After looking closer at the problem I was able to reduce it to a small C 
program. (Program is attached.)

It looks like FPU registers get sometimes lost after switching between 
two or more threads.
It also happens not always, that's why my test program contains a 
infinite loop. After a few million iterations the program abort()s.

I can reproduce the issue on both x86 and x86_64, the host's or UML's 
kernel version does not matter.
I've tested 2.6.18 to 3.1-rc2.

Interestingly the problem occurs not on my old Pentium 4 machines.
One P4 has HT the other not.
Only "newer" CPUs are affected.

First I thought it's a race in _switch_to(), but adding 
unblock/block_signals() did not help.
Currently I'm running out of ideas.
I'm not an expert in this area of UML. :-(

Any idea what goes wrong here?

Thanks,
//richard

View attachment "fpu-test.c" of type "text/x-csrc" (448 bytes)