HALT! on Linux

From the victim:
   Linux won't load on my 9595-SOC.. (P90) The boot freeze at the "Checking for HLT command"


From Rick Hohensee
   HLT is a Pentium-and-up instruction Linux uses to idle if it's available. A 386 kernel won't even look for it.


From Dr. Jim:
   So the question remains- why does the HLT freak out a Pentium box?

I beleive that question was answered:

> The problem hinges around the fact that the FPU test disables the timer interrupt.
> No idea why.


From Dr. Jim:
   I'm not sure where, but I think I saw a mention of the P90 as having >some< FDIVd steppings.

From http://www.intel.com/procs/support/pentium/fdiv/replqa3.htm:
Q3: What speeds of the Pentium processor are affected by the floating point flaw?
A3: If you have a 60, 66, 75, 90 or 100MHz Pentium processor, it is possible you may be affected with the floating point flaw. To check if you are affected, use CPUID. Any speeds higher than 100MHz do not have the FPU flaw.

Don´t blame me, but sorry Jim, but you are wrong. Direct from the kernel-sources:


From Stephan Goll:

if (mca_pentium_flag) {
                /* The IBM Model 95 machines with pentiums lock up on
                 * fpu test, so we avoid it. All pentiums have inbuilt
                 * FPU and thus should use exception 16. We still do
                 * the FDIV test, although I doubt there where ever any
                 * MCA boxes built with non-FDIV-bug cpus.
---------
printk("mca-pentium specified, avoiding FPU coupling test... ");
        if (!boot_cpu_data.fdiv_bug)
                printk("??? No FDIV bug? Lucky you...\n");
        else
                printk("detected FDIV bug though.\n");
        return;
        }
---------

See bugs.h for more details.

Greetings,
Stephan


From Louis Ohland:
   OK, campers, this is real answers. Maybe not the full truth, but answers.

Exception 16?
   The snippet of code inferrs that it was intended to test for the bug, and give either thumbs up or down, but not lock up. Why does (!boot_cpu_data.fdiv_bug) fail?


From Su:
   I'd tend to agree with Jim's assessment that it's the *test* that causes the hang. I found this in 'bugs.h':

        /*
         * check if exception 16 works correctly.. This is truly evil
         * code: it disables the high 8 interrupts to make sure that
         * the irq13 doesn't happen. But as this will lead to a lockup
         * if no exception16 arrives, it depends on the fact that the
         * high 8 interrupts will be re-enabled by the next timer tick.
         * So the irq13 will happen eventually, but the exception 16
         * should get there first..
         */

This comment is in a fuction that is called before the one that says it's "Checking 'hlt' instruction...". I'm wondering if the high 8 interrupts get re-enabled (and therefore the IRQ 13) *before* the exception 16 arrives, and when the asm code that actually does the HLT is executed, the system hangs.

I haven't been through all the code yet, just bugs.h, so this is just a guess...


From Dr. Jim:
   What I'm wondering is if it isn't another 'gotcha' resulting from the fact that MCA interrupts are level-triggered rather than edge-triggered. If a level-triggered interrupt remains asserted, and
you are looking for an edge-trigger, you will never see it. Tony? Peter? Comments?


What to Do?

From Dr. Jim Shorney:
   Is there a parameter you can pass to the bootdisk?
At the boot prompt: yourkernelname mca-pentium

Content created and/or collected by:
Louis Ohland, Peter Wendt, William Walsh, Kevin Bowling, Jim Shorney, Tim Clarke, David Beem, Tatsuo Sunagawa, Tomáš Slavotínek, and many others.

Ardent Tool of Capitalism - MAD Edition! is maintained by Tomáš Slavotínek.
Last update: 17 Apr 2021 - Changes & Credits | Legal Info & Contact