Gitweb:
http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=12c247a6719987aad65f83158d2bb3e73c75c1f5
Commit: 12c247a6719987aad65f83158d2bb3e73c75c1f5
Parent: 03994f01e8b72b3d01fd3d09d1cc7c9f421a727c
Author: Mikael Pettersson <[EMAIL PROTECTED]>
AuthorDate: Sun Feb 24 18:27:03 2008 +0100
Committer: Ingo Molnar <[EMAIL PROTECTED]>
CommitDate: Tue Feb 26 12:56:04 2008 +0100
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.
[ [EMAIL PROTECTED]: fixed a similar bug in setup_64.c as well. ]
The breakage was introduced via commit 7d851c8d3db0.
Signed-off-by: Mikael Pettersson <[EMAIL PROTECTED]>
Signed-off-by: Ingo Molnar <[EMAIL PROTECTED]>
---
arch/x86/kernel/cpu/common.c | 2 +-
arch/x86/kernel/setup_64.c | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index f86a3c4..a38aafa 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -504,7 +504,7 @@ 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];
/* Init Machine Check Exception if available. */
mcheck_init(c);
diff --git a/arch/x86/kernel/setup_64.c b/arch/x86/kernel/setup_64.c
index 6fd804f..7637dc9 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);
-
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