Mikael Pettersson writes: > Ingo Molnar writes: > > > > * Mikael Pettersson <[EMAIL PROTECTED]> 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 <[EMAIL PROTECTED]> --- 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); -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/