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SubjectRe: UP local APIC is deadly on SMP Athlon
On Sat, 22 Feb 2003, Mikael Pettersson wrote:

> This is interesting. Very interesting, even. A plain UP_APIC kernel
> (with IO_APIC not enabled or not detected) shouldn't need to touch
> APIC_ID at all. I strongly suspect this is a remnant of apic.c's old
> SMP-only history, and that it should be removed for UP_APIC-only.
> I need to get some downtime (zzz...) but I'll look into this ASAP.

More testing on more platforms actually lead me to a slightly different
patch, which makes a lot more sense as far as phys_cpu_present_map is

--- linux-2.4.21-pre4/arch/i386/kernel/apic.c.old Fri Jan 31 10:32:12 2003
+++ linux-2.4.21-pre4/arch/i386/kernel/apic.c Sat Feb 22 02:47:02 2003
@@ -1169,8 +1169,8 @@


- phys_cpu_present_map = 1;
- apic_write_around(APIC_ID, boot_cpu_physical_apicid);
+ phys_cpu_present_map = (1 << boot_cpu_physical_apicid);
+ printk("Setting %d in the phys_cpu_present_map\n", boot_cpu_physical_apicid);


This has passed my testing on the following platforms:

* P3 (I820 chipset, no IO-APIC, APIC originally disabled by BIOS)
* P3 (440BX chipset, no IO-APIC, APIC originally disabled by BIOS)
* single P3 (ServerWorks OSB4 chipset, one CPU in dual CPU MB)
* dual P3 (ServerWorks OSB4 chipset, both CPU's present)
* dual P4 Xeon (I7500 chipset, both CPU's present, HT enabled)
* K7 (VIA KT400 chipset, IO-APIC present)
* K7 (VIA KM133 chipset, IO-APIC present)
* dual K7 (AMD 760MP chipset, both CPU's present)
* dual K7 (AMD 760MPX chipset, both CPU's present)

As a matter of fact, I got very interesting numbers from that printk() I

- all the Intel and single proc AMD printed "0".
- all the dual AMD machines printed "1".

So the reason it was crashing on the dual Athlons is two-fold:

- It would try to write 1 into APIC_ID, when instead it should have
written (1 << 24). So it was effectively setting the APIC ID to 0.
- It would unconditionally set bit 0 in the phys_cpu_present_map bitmap,
but later on it would check bit number boot_cpu_physical_apicid and BUG()
if it found it clear.

So I think the patch above is safe. We could maybe replace the
old apic_write_around() with something like

apic_write_around(APIC_ID, (boot_cpu_physical_apicid << 24))

but it's probably unnecessary, as you said.

Ok. Enough for today. Zzzz catching time for me too...


It is better to keep your mouth shut and be thought a fool,
than to open it and remove all doubt.

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