This is one of the bugs, among others, we have fixed. I'm not sure, if Ralf have integrated the patches we send him yet. /Carsten Louis Hamilton wrote: > 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> -- _ _ ____ ___ Carsten Langgaard Mailto:carstenl@mips.com |\ /|||___)(___ MIPS Denmark Direct: +45 4486 5527 | \/ ||| ____) Lautrupvang 4B Switch: +45 4486 5555 TECHNOLOGIES 2750 Ballerup Fax...: +45 4486 5556 Denmark http://www.mips.com
