kernel panic despite enabling math-emulation

From: Reza Mostafid
Date: Sun May 31 2009 - 01:41:03 EST

I am trying to build a stripped down v2.6.* kernel to run on a single
chip 386 processor.
As I understand it, the math emulation library must be enabled (
compiled into the kernel )
to cater for the absence of a proper HW FPU.

I am doing the builds on a more modern x86 desktop machine running Fedora 8
( based on kernel ) using the source from Fedora 8 ( i.e.
with the F8 patches
applied against a kernel )

As a first step, after building the kernel with
'CONFIG_MATH_EMULATION=y' I tried to test
this feature on my build machine ( which does have an FPU ), by
supplying the 'no387'
boot parameter in order to simulate a non-existing FPU.

Running the math-emu enabled kernel in this way causes a kernel panic
which happens before
spawning 'init' and early-user space initialization.

I have observed this condition with several 2.6.* kernel versions and
several Intel/AMD boxes.

Using the 'earlyprintk=vga' bootparameter and some printk statements
sprinkled in the
appropriate corners of the code ( i.e. '__init' function 'no387()' in
bug.c ) I was able to
observe that in all three cases the 'no387()' code snippet executes
its stuff without hick ups
when the 'no388' boot parameter is processed

37 static int __init no_387(char *s)
38 {
39 boot_cpu_data.hard_math = 0;
40 write_cr0(0xE | read_cr0());
41 return 1;
42 }
44 __setup("no387", no_387);

The kernel freeze happens as a result of a panic in 'fpu_entry.c' (
here I have reproduced code
and line no.s from the file as it exists in v2.6.23.1 under
'arch/i386/math-emu' )

179 else if ( FPU_CS == __KERNEL_CS )
180 {
181 printk("math_emulate: %04x:%08lx\n",FPU_CS,FPU_EIP);
182 panic("Math emulation needed in kernel");
183 }
184 else

In all cases the following lines are the last outputs to the console:

math_emulate: <some FPU_CS value>:<some FPU_EIP value>
Kernel panic-not syncing: Math emulation needed in kernel

So it appears that even with math emulation enabled the logic of the
code in 'fpu_entry.c' takes
the kernel into the above 'else', i.e. the kernel thinks that there is
no 'math emulation'.

Searching the web I was able to find references to this problem in
several threads but no apparent
solution to it.

Math-emulation albeit not a widely used feature, should have been
around long enough for someone to
have stumbled upon such an obvious problem.

Q1) Am I overlooking anything?


Q2) Is this a problem in the code?

and finally

Q3) Any suggestions for how I could get a 2.6 kernel to actually run
with the math-emu?


Reza Mostafid


Here is a reproduction of the steps I took:

i) I downloaded the source rpms for the Fedora distro I am running (
Fedora Core 8 which runs
a modified version of the kernel ).

ii) I compiled the Fedora kernel version thus obtained ( using the
supplied distro .config ) and
it boots fine

iii) I then change a single parameter of the original configuration,
i.e. enable math-emulation and
I observed the problem ( kernel panics with the 'no387' param
supplied )

iv) I then reproduced the same problem booting a '' vanilla kernel with the same
configuration and observed the same problem.

v) Finally several different versions of the '' vanilla
source were compiled with the same
configuration ( the FC8 distro '.config' with just the 'math-emu'
enabled ) on three different PCs.
Each time the same problem: Each math-emu enabled kernel boots
fine, until you supply the 'no387'
option, in which case all of them hang at the same place.

Configuration used as base: That of the Fedora 8 distro

Parameter changed: CONFIG_MATH_EMULATION=y

Kernels tested: source rpm for Fedora 8 'vanilla' from '' 'vanilla' from '' 'vanilla' from '' 'vanilla' from ''

CPUs/machines problem was observed on:

Intel Celeron 331 ( Netburst type P4 )
AMD Semperon
AMD Athlon
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