Claas Langbehn wrote:
> Could anyone explain to me what CMPXCHG64 / cx8 is and what happens if
> the kernel has been compiled to use it but the CPU does not have it?
>
> Regards,
> claas
>
Hi Claas,
the bug is at another place. But it's hidden if X86_CMPXCHG64 is
switched of.
There are 3 errors.
First: Via is capable of CMPXCHG64, but switch the related CPU bit off
to work with old Win NT versions. ( So the NT error is the fourth... but who
cares
about that error. ;-) )
Second: verify_cpu.S
This checks during the boot the capability of the CPU. It expected
the related bit, but... see first.
This error is switched of if X86_CMPXCHG64 is disabled.
Third: setup.S "PANIC: CPU too old for this kernel." is not printed
cause of a missing set register in setup.S.
As I have learned the NX-bit of the C7 is a BIOS issue.
The only real required patch is below. It enables the right bits for the C3/C7.
The first error is out of scope, and the fix for the "CPU too old.." is for
me just a cosmetic print out in case of boot errors.
I suppose .rc3 should contain a fix for this bug.
Best regards,
Christian
--- linux.orig/arch/i386/kernel/verify_cpu.S
+++ linux/arch/i386/kernel/verify_cpu.S
@@ -2,6 +2,7 @@
This runs in 16bit mode so that the caller can still use the BIOS
to output errors on the screen */
#include <asm/cpufeature.h>
+#include <asm/msr.h>
verify_cpu:
pushfl # Save caller passed flags
@@ -45,6 +46,28 @@
cmpl $0x1,%eax
jb bad # no cpuid 1
+#if REQUIRED_MASK1 & NEED_CMPXCHG64
+ /* Some VIA C3s need magic MSRs to enable CX64. Do this here */
+ cmpl $0x746e6543,%ebx # Cent
+ jne 1f
+ cmpl $0x48727561,%edx # aurH
+ jne 1f
+ cmpl $0x736c7561,%ecx # auls
+ jne 1f
+ movl $1,%eax # check model
+ cpuid
+ shr $4,%eax
+ andl $0xf,%eax # get model
+ cmpl $6,%eax
+ jb 1f
+ cmpl $10,%eax # newer vias hopefully don't require
+ ja 1f # this anymore
+ movl $MSR_VIA_FCR,%ecx
+ rdmsr
+ orl $((1<<1)|(1<<7)),%eax # enable CMPXCHG64 and PGE
+ wrmsr
+1:
+#endif
movl $0x1,%eax # Does the cpu have what it takes
cpuid
-
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