Message-ID: <464CC8E1.5010407@cv-sv.de>
Date: Thu, 17 May 2007 23:28:01 +0200
From: Christian Volkmann <haveaniceday@cv-sv.de>
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CC: Dave Jones <davej@redhat.com>, linux-kernel@vger.kernel.org, ak@suse.de,
       Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: 2.6.22-rc1 does not boot on VIA C3_2 cause of X86_CMPXCHG64
References: <464B9D2C.7040704@cv-sv.de> <20070517004209.GE16810@redhat.com> <464BB252.8010008@cv-sv.de>
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Christian wrote:
Hmm, I really think so...:
> 
> May I brought up a wrong reason with the command cmpxchg64.
> But disabling CONFIG_X86_CMPXCHG64 helps.
> 


Hi,


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,

Christian

--- 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. */
+#if CONFIG_MVIAC3_2
+       andl    $~NEED_CMPXCHG64,%edx
+#endif
+
        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

 cpu_panic_mess:
-       .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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