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? TIA, Louis Louis Hamilton hamilton@redhat.com ------ 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 mips_float_debug_loop: li $9, 0xF000F02F ctc1 $9, $31 # set FCR to some non-zero value nop 1: cfc1 $8, $31 # get FCR beq $8, $9, 1b # spin, waiting for FCR to change nop or $2, $0, $8 jr $31 nop 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. <end>
