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SubjectFloating point problems on UML - help needed

Gunnar reported that some Java program does not work proper within
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?

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

static int xxx(float f)
if(f <= 0.0){
printf("wrong f!: %f\n", f);
return -1;

return 0;

static void *fun(void *arg)
float f = 5.0;

if(xxx(f) < 0)

return NULL;

int main()
pthread_t t1, t2;

pthread_create(&t1, NULL, fun, NULL);
pthread_create(&t2, NULL, fun, NULL);

pthread_join(t1, NULL);
pthread_join(t2, NULL);

return 0;
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