Subject: "Setting flush to zero for ..." - what is the warning?

From: Jun Sun <[email-masked]>
Date: Wed, 01 Nov 2000 10:58:13 -0800

I ran some stress tests and start to get this warning.  It appears to be
generated in do_fpe() routine.  See below.  I wonder why this is
happening.  Can someone explain what is going on?  Thanks.

Jun

void do_fpe(struct pt_regs *regs, unsigned long fcr31)
{
        ....
        if (fcr31 & 0x20000) {
                /* Retry instruction with flush to zero ...  */
                if (!(fcr31 & (1<<24))) {
                        printk("Setting flush to zero for %s.\n",
                               current->comm);
                        fcr31 &= ~0x20000;
                        fcr31 |= (1<<24);
                        __asm__ __volatile__(
                                "ctc1\t%0,$31"
                                : /* No outputs */
                                : "r" (fcr31));
                        return;
                }

---

From: Ralf Baechle <[email-masked]>
Date: Thu, 2 Nov 2000 04:06:58 +0100

> happening.  Can someone explain what is going on?  Thanks.

It tells you the over-the-thumb-fp-mode has been activated ;-)

Somebody at MIPS is working on merging the necessary fp support software
into the kernel, so this problem should be solved soon.

  Ralf

---

From: "Kevin D. Kissell" <[email-masked]>
Date: Fri, 3 Nov 2000 11:58:56 +0100

> It tells you the over-the-thumb-fp-mode has been activated ;-)

More seriously, there was (is, in 2.4 I guess) a hack by which,
in a desperate attempt to avoid having to do the FP emulation
in software, the kernel changed the FPU denorm handling mode
and replayed the instruction, in hopes that the problem would
go away (which it would for a subset of the unimplemented
operation cases). This is not legal IEEE behaviour, as it turns out,
but not many people cared.

> into the kernel, so this problem should be solved soon.

Once we had bolted the Algorithmics FPU emulator into the kernel,
the hack was no longer necessary.   To say that "somebody at MIPS
is working on merging the necessary fp support software into the
kernel" is perhaps a bit misleading.  The FPU emulator itself is in
the oss.sgi.com repository, in the 2_2 branch, but I did not merge
in the hacks to the kernel exception, context, signal, etc. handling.
And there are several bug fixes that have been made since then.
All the additional code is available on the ftp.mips.com server, and
has been merged by others into 2.3/2.4, most notably by the VrLinux
guys.

We've got 2.4-test running in the lab, but it is a long way
from being as robust under torture as our 2.2 kernel, and
we have not decided whether it is "ripe" enough to merge
in the FPU emulation support ourselves.

            Regards,

            Kevin K.

---

From: Jun Sun <[email-masked]>
Date: Fri, 03 Nov 2000 09:55:31 -0800

> but not many people cared.

I am reading between the lines.  Do you mean

1) even though the CPU (R5432 in this case) has a FPU, some instructions
(or under certain conditions) are NOT supported by the hardware?
2) So in those cases, software should do the job, but the existing 2.4
is not doing it right?
3) Can we summarize exactly what instructions (under what conditions)
are considered not supported by hardware?  Or is it too complicated to
summarieze in short?  Or should it be documented in CPU manual (which
may vary for different CPUs)

> has been merged by others into 2.3/2.4, most notably by the VrLinux

If I understand correctly, FPU emulator is for the case where FPU is
completely absent.  Does it do the job we are talking about here?

Thanks.

Jun  
... totally ignorant about FPU

---

From: "Kevin D. Kissell" <[email-masked]>
Date: Fri, 3 Nov 2000 19:55:13 +0100

> (or under certain conditions) are NOT supported by the hardware?

That is correct.  See (3) below.

> 2) So in those cases, software should do the job, but the existing 2.4

That is sort-of correct.  The mini-emulator in the existing distribution
will handle a subset of the common cases correctly.  The flush-to-zero
trick avoids another subset, though not stricktly speaking correctly.

> may vary for different CPUs)

Architecturally, a MIPS FPU is always allowed to deliver an
Unimplemented Operation instruction if the combination of
instruction and operand values goes beyond what it can deal
with.  In theory, that means that it would be legal to simply not
implement certain difficult instructions altogether (I have no
examples of this) or to implement single-precision but not
double-precision operands (this, I believe, has been done).
But even in full-blown implementations like the R4000/4400
or R5000 or R5432, there are often corner cases that the
designers considered too expensive to handle in hardware.
In general, these fall into two categories: conversion operations
on extreme values that exceed the size of the shifter in the
FPU, and operations on denormalized values - according
to the IEEE standard, 2.0 denorm + 2.0 denorm = 4.0 denorm,
but it's a rare RISC FPU that will really perform that computation.
Setting flush-denormals-to-zero can thus avoid these exceptions,
but won't provide the values stipulated by the IEEE spec, either.

> completely absent.  Does it do the job we are talking about here?

We integrated the Algorithmics FPU emulator because there
are embedded MIPS chips out there (the MIPS 4Kc/m/p and 5K,
the NEC Vr41xx, the Toshiba Tx49) that have no FPUs and really
need the full emulation.  But while it may seem like overkill, having the
full emulator does cover the unimplemented corner cases very
nicely.  With a couple of fixes elsewhere (the kernel branch emulation
had/has a signed/unsigned bug), we finally have MIPS/Linux platforms
that pass the "paranoia" IEEE FP compliance test 100%, with and
without FPUs.

            Regards,

            Kevin K.