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SubjectRe: [BUG] 2.6.25-rc2-git8 fails to boot on 486 due to TSC breakage
Mikael Pettersson writes:
> Ingo Molnar writes:
> >
> > * Mikael Pettersson <mikpe@it.uu.se> wrote:
> >
> > > The kernel for this 486 has CONFIG_M486=y and CONFIG_M586TSC=n, but
> > > the 2.6.25 kernels still try to access the TSC. Here's the oops from
> > > 2.6.25-rc2-git8:
> >
> > hm, could you send me the full .config you used?
>
> I've put it here:
> <http://user.it.uu.se/~mikpe/linux/tmp/config-2.6.24-git8>
>
> Meanwhile, I've traced the breakage to 2.6.24-git8.
>
> 2.6.24-git8 changed include/asm-x86/tsc.h:get_cycles() to call
> rdtscll() even if CONFIG_X86_TSC isn't set. The call is protected
> by a cpu_has_tsc test, but starting with 2.6.24-git8 cpu_has_tsc
> is non-zero on this machine, which is very very wrong.
>
> Diffing dmesg between git7 and git8 doesn't sched any light since
> git8 also removed the printouts of the x86 caps as they were being
> initialised and updated. I'm currently adding those printouts back
> in the hope of seeing where and when the caps get broken.

That turned out to be very illuminating:

--- dmesg-2.6.24-git7 2008-02-24 18:01:25.295851000 +0100
+++ dmesg-2.6.24-git8 2008-02-24 18:01:25.530358000 +0100
...
CPU: After generic identify, caps: 00000003 00000000 00000000 00000000 00000000 00000000 00000000 00000000
CPU: After all inits, caps: 00000003 00000000 00000000 00000000 00000000 00000000 00000000 00000000
+CPU: After applying cleared_cpu_caps, caps: 00000013 00000000 00000000 00000000 00000000 00000000 00000000 00000000
Notice how the TSC cap bit goes from Off to On.

(The first two lines are printout loops from -git7 forward-ported
to -git8, the third line is the same printout loop added just after
the xor-with-cleared_cpu_caps[] loop.)

Here's how the breakage occurs:
1. arch/x86/kernel/tsc_32.c:tsc_init() sees !cpu_has_tsc,
so bails and calls setup_clear_cpu_cap(X86_FEATURE_TSC).
2. include/asm-x86/cpufeature.h:setup_clear_cpu_cap(bit) clears
the bit in boot_cpu_data and sets it in cleared_cpu_caps
3. arch/x86/kernel/cpu/common.c:identify_cpu() XORs all caps
in with cleared_cpu_caps
HOWEVER, at this point c->x86_capability correctly has TSC
Off, cleared_cpu_caps has TSC On, so the XOR incorrectly
sets TSC to On in c->x86_capability, with disastrous results.

The real bug is that clearing bits with XOR only works if the
bits are known to be 1 prior to the XOR, and that's not true here.

A simple fix is to convert the XOR to AND-NOT instead. The following
patch does that, and allows my 486 to boot 2.6.25-rc kernels again.

Signed-off-by: Mikael Pettersson <mikpe@it.uu.se>
---
There's a similar XOR loop in arch/x86/kernel/setup_64.c.
I haven't seen it fail yet, but perhaps it should be changed
too, for robustness and symmetry.
--- linux-2.6.25-rc2-git8/arch/x86/kernel/cpu/common.c.~1~ 2008-02-24 17:42:56.000000000 +0100
+++ linux-2.6.25-rc2-git8/arch/x86/kernel/cpu/common.c 2008-02-24 17:44:06.000000000 +0100
@@ -504,7 +504,7 @@ void __cpuinit identify_cpu(struct cpuin

/* Clear all flags overriden by options */
for (i = 0; i < NCAPINTS; i++)
- c->x86_capability[i] ^= cleared_cpu_caps[i];
+ c->x86_capability[i] &= ~cleared_cpu_caps[i];

/* Init Machine Check Exception if available. */
mcheck_init(c);

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