On Wed, Nov 28, 2012 at 04:14:43PM -0800, H. Peter Anvin wrote:
> Wrong barrier semantics.
Let me try to understand what you mean by that :)
Now, your version's asm output looks like this:
----
.loc 2 95 0
movl $139, %esi #, tmp140
xorl %eax, %eax # tmp141
movl %esi, %ecx # tmp140,
movl %eax, %edx # tmp141,
#APP
# 95 "/home/boris/kernel/linux-2.6/arch/x86/include/asm/msr.h" 1
wrmsr
# 0 "" 2
#NO_APP
.loc 3 694 0
movb $1, %al #,
#APP
# 694 "/home/boris/kernel/linux-2.6/arch/x86/include/asm/processor.h" 1
cpuid
# 0 "" 2
.LVL15:
#NO_APP
----
This is the sync_core() call from early_init_intel(). Now look like 139
remains in %ecx after the WRMSR and CPUID actually gets called with
RAX=1 and RCX=139 (btw, 139 is MSR_IA32_UCODE_REV). Even if this works
I'd say, we don't want to have any stray values in RCX when doing CPUID,
no?
Now here's the version with the change I proposed:
----
.loc 2 95 0
xorl %esi, %esi # tmp144
movl $139, %edi #, tmp143
movl %edi, %ecx # tmp143,
movl %esi, %eax # tmp144,
movl %esi, %edx # tmp144,
#APP
# 95 "/home/boris/kernel/linux-2.6/arch/x86/include/asm/msr.h" 1
wrmsr
# 0 "" 2
.LVL15:
#NO_APP
.loc 3 199 0
movb $1, %al #,
movl %esi, %ecx # tmp144, ecx
#APP
# 199 "/home/boris/kernel/linux-2.6/arch/x86/include/asm/processor.h" 1
cpuid
# 0 "" 2
.LVL16:
#NO_APP
----
RCX gets correctly cleaned to 0 and *then* we call CPUID.
And the asm output is the same except that RCX gets correctly cleaned up
before calling CPUID.
So what am I missing?
Thanks.
--
Regards/Gruss,
Boris.
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