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SubjectRe: 2.6.22-rc1 does not boot on VIA C3_2 cause of X86_CMPXCHG64
Christian wrote:
Hmm, I really think so...:
> May I brought up a wrong reason with the command cmpxchg64.
> But disabling CONFIG_X86_CMPXCHG64 helps.


I found some time to investigate. My resume is:

- kernel/verify_cpu.S causes the stop at boot time
Cause the required flag for CMPXCHG8B is not set.

- boot/setup.S did not print "PANIC: CPU too old for this kernel"
( not visible, also the message did not match )

- VIA C3_2: CMPXCHG8B should work always. Via C3 EBGA Datasheet R1.90 page 30 (3-4)
But the bit is not set cause of some early Win NT bugs.

- X86_CMPXCHG64 can be set and used. Just verify_cpu.S requires a change
to ignore the missing indicator at boot time.

My suggestion for the fix is:

- kernel/verify_cpu.S should be more tolerant if CONFIG_MVIAC3_2 is set
( see patch below )

- boot/setup.S should be fixed to print out a proper error message
( see below )

- X86_CMPXCHG64 patch(earlier mail in thread ) should not be used.

- Important: somebody to check other CPU types if the same behavior happens.

- middle term solution already planed: introduce arch/i386/boot/cpucheck.c

To be done by somebody with more detail knowledge than me: (never did x86 assembler before)

- I add "# missed before: set ds"
=> somebody should check if I am right with the way to set.
=> seems to be a generic error in setup.S not to set "ds" for error messages.

- other X86-CPU than AMD (?) or Intel
=> correct verify_cpu.S or set bits if required.

Best regards,


--- arch/i386/kernel/verify_cpu.S 2007-05-17 05:17:40.000000000 +0200
+++ arch/i386/kernel/verify_cpu.S 2007-05-17 23:14:18.266679323 +0200
@@ -54,6 +54,12 @@

andl $REQUIRED_MASK1,%edx
xorl $REQUIRED_MASK1,%edx
+/* VIAC3 does not report the existing CMPXCHG64 by default. So do not go to bad for CMPXCHG64 for this */
+/* via C3 EBGA datasheet R1.9 tells the command works also without shown bit. */
+ andl $~NEED_CMPXCHG64,%edx
jnz bad
#endif /* REQUIRED_MASK1 */

--- arch/i386/boot/setup.S 2007-05-17 21:53:08.009226208 +0200
+++ arch/i386/boot/setup.S 2007-05-17 23:12:36.815989168 +0200
@@ -310,12 +310,15 @@
call verify_cpu
testl %eax,%eax
jz cpu_ok
+ # missed before: set ds
+ movw %cs, %ax # aka SETUPSEG
+ movw %ax, %ds
lea cpu_panic_mess,%si
call prtstr
1: jmp 1b

- .asciz "PANIC: CPU too old for this kernel."
+ .asciz "PANIC: CPU too old for this kernel or CPUID function bits are wrong."

#include "../kernel/verify_cpu.S"

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