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 22.214.171.124 kernel ) using the source from Fedora 8 ( i.e.
with the F8 patches
applied against a 126.96.36.199 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)
39 boot_cpu_data.hard_math = 0;
40 write_cr0(0xE | read_cr0());
41 return 1;
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 v188.8.131.52 under
179 else if ( FPU_CS == __KERNEL_CS )
181 printk("math_emulate: %04x:%08lx\n",FPU_CS,FPU_EIP);
182 panic("Math emulation needed in kernel");
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?
Q3) Any suggestions for how I could get a 2.6 kernel to actually run
with the math-emu?
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 184.108.40.206 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
iv) I then reproduced the same problem booting a 'kernel.org' vanilla
220.127.116.11 kernel with the same
configuration and observed the same problem.
v) Finally several different versions of the 'kernel.org' 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
18.104.22.168 source rpm for Fedora 8
22.214.171.124 'vanilla' from 'kernel.org'
126.96.36.199 'vanilla' from 'kernel.org'
188.8.131.52 'vanilla' from 'kernel.org'
184.108.40.206 'vanilla' from 'kernel.org'
CPUs/machines problem was observed on:
Intel Celeron 331 ( Netburst type P4 )
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