Messages in this thread Patch in this message | | | Date | Mon, 25 Feb 2008 10:19:55 +0100 | From | Ingo Molnar <> | Subject | Re: [BUG] 2.6.25-rc2-git8 fails to boot on 486 due to TSC breakage |
| |
* Mikael Pettersson <mikpe@it.uu.se> wrote:
> > yes, i already fixed that when i added Mikael's patch and it's all > > queued up. > > Ok. For reference and for LKML viewers, this is what the final patch > should be:
below is the full commit - will send it to Linus later today.
Ingo
---------------> Subject: x86: fix boot failure on 486 due to TSC breakage From: Mikael Pettersson <mikpe@it.uu.se> Date: Sun, 24 Feb 2008 18:27:03 +0100
> 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> --- arch/x86/kernel/cpu/common.c | 2 +- arch/x86/kernel/setup_64.c | 2 +- 2 files changed, 2 insertions(+), 2 deletions(-)
Index: linux-x86.q/arch/x86/kernel/cpu/common.c =================================================================== --- linux-x86.q.orig/arch/x86/kernel/cpu/common.c +++ linux-x86.q/arch/x86/kernel/cpu/common.c @@ -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); Index: linux-x86.q/arch/x86/kernel/setup_64.c =================================================================== --- linux-x86.q.orig/arch/x86/kernel/setup_64.c +++ linux-x86.q/arch/x86/kernel/setup_64.c @@ -1021,7 +1021,7 @@ 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]; #ifdef CONFIG_X86_MCE mcheck_init(c);
| |