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authorMikael Pettersson <mikpe@it.uu.se>2008-02-24 18:27:03 +0100
committerIngo Molnar <mingo@elte.hu>2008-02-26 12:56:04 +0100
commit12c247a6719987aad65f83158d2bb3e73c75c1f5 (patch)
tree6cb18d66fe259e3023f81cdf1cc6f6728a216564 /arch/x86/kernel/setup_64.c
parent03994f01e8b72b3d01fd3d09d1cc7c9f421a727c (diff)
x86: fix boot failure on 486 due to TSC breakage
> 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 7d851c8d3db0. Signed-off-by: Mikael Pettersson <mikpe@it.uu.se> Signed-off-by: Ingo Molnar <mingo@elte.hu>
Diffstat (limited to 'arch/x86/kernel/setup_64.c')
-rw-r--r--arch/x86/kernel/setup_64.c2
1 files changed, 1 insertions, 1 deletions
diff --git a/arch/x86/kernel/setup_64.c b/arch/x86/kernel/setup_64.c
index 6fd804f07821..7637dc91c79b 100644
--- a/arch/x86/kernel/setup_64.c
+++ b/arch/x86/kernel/setup_64.c
@@ -1021,7 +1021,7 @@ void __cpuinit identify_cpu(struct cpuinfo_x86 *c)
/* 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];
#ifdef CONFIG_X86_MCE
mcheck_init(c);