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Bug in Linux? fcr31 not being saved-restored

Subject: Bug in Linux? fcr31 not being saved-restored
From: Louis Hamilton <>
Date: Sat, 15 Jun 2002 18:56:30 -0500
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We have a customer here testing a 2.4.16 mips kernel on an embedded
Linux RM7000/SR71000 based system who has written a test that they
believe has uncovered a bug in Linux.  The FPU control register appears
to not get saved and restored.  I've reproduced the problem described
below and find the results consistent with their description.  The
problem occurs on both the RM7000 and SR71000 cpus.

It looks like save_fp_context and restore_fp_context are not being
called since the kernel save-restore logic thinks the process is not using floating point math. If you do some fp math before calling the
test routine below, it seems to works fine.

Is this a known caveat?  A true bug?  Or a contorted corner case
unlikely to be seen under typical end-user usage (see customer's
last paragraph :-) ?   If true bug, recommended remedy?


Louis Hamilton

------ customer reports the following: ---------
We found a bug in Linux.  A ^C (control-C) typed into a shell (or a
running program, it doesn't matter), causes the FCR (floating-point
control register) to be corrupted in another, unrelated process.  This
is repeatable behavior.

This can be reproduced with the following short assembly language
program that loops forever, waiting for the FCR to change.

        .align 2
        .globl mips_float_debug_loop
        li      $9, 0xF000F02F
        ctc1    $9, $31         # set FCR to some non-zero value
1:      cfc1    $8, $31         # get FCR
        beq     $8, $9, 1b      # spin, waiting for FCR to change
        or      $2, $0, $8
        jr    $31

You can call this function from a short C program and the return value
is the (corrupted) FCR, which turns out to alwyas be: 0x00000002.

Run the above loop in one window (connected to the board using telnet)
and then in another window (connected to the same board) type ^C.

I'm surprised this bug hasn't been encountered by other MIPS vendors.


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