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x86: fix boot failure on 486 due to TSC breakage

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> 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.

[ mingo@elte.hu: fixed a similar bug in setup_64.c as well. ]

The breakage was introduced via commit 7d851c8d3d.

Signed-off-by: Mikael Pettersson <mikpe@it.uu.se>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Mikael Pettersson 2008-02-24 18:27:03 +01:00 committed by Ingo Molnar
parent 03994f01e8
commit 12c247a671
2 changed files with 2 additions and 2 deletions
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2
arch/x86/kernel/cpu/common.c
View File

@ -504,7 +504,7 @@ void __cpuinit identify_cpu(struct cpuinfo_x86 *c)
/* Clear all flags overriden by options */ /* Clear all flags overriden by options */
for (i = 0; i < NCAPINTS; i++) 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. */ /* Init Machine Check Exception if available. */
mcheck_init(c); mcheck_init(c);

2
arch/x86/kernel/setup_64.c
View File

@ -1021,7 +1021,7 @@ void __cpuinit identify_cpu(struct cpuinfo_x86 *c)
/* Clear all flags overriden by options */ /* Clear all flags overriden by options */
for (i = 0; i < NCAPINTS; i++) for (i = 0; i < NCAPINTS; i++)
c->x86_capability[i] ^= cleared_cpu_caps[i]; c->x86_capability[i] &= ~cleared_cpu_caps[i];
#ifdef CONFIG_X86_MCE #ifdef CONFIG_X86_MCE
mcheck_init(c); mcheck_init(c);