[OpenBIOS] [PATCH] ppc: fix mtmsr for SF setting

Segher Boessenkool segher at kernel.crashing.org
Fri Jun 17 02:05:30 CEST 2011

>>>>> 	mfmsr	r1 ;				/* unset MSR_SF */ \
>>>>> 	clrlwi	r1,r1,0 ; \
>>>>> -	mtmsr	r1 ; \
>>>>> +	mtmsrd	r1 ; \
>>>> clrlwi 1,1,0  is  rlwinm 1,1,0,0,31  which clears all the top 32 
>>>> bits, not
>>>> only MSR[SF].  Importantly it clears MSR[HV], which you do not 
>>>> want.  Use
>>>> rldicl instead?
>>> Now that we actually detect if we're on a 64-bit CPU, we can indeed 
>>> be a bit more clever about it. However, I don't think any of the 
>>> upper 32 bits are important to us, no?
>> You set MSR[HV]=0.  That will cause trouble: many mtspr's are then 
>> treated as
>> a noop, for example.  And if you have an RMO, things break.  Etc.  
>> Trouble :-)
> But we don't run on any of those, do we? You're probably right though 
> - let's make this clean.

You probably don't set real mode offset in the firmware, no -- but the 
SPR thing
will kill you.  In non-hypervisor mode you cannot write most of the fun 

>>>> lis x,0x8000 ; add. x,x,x ; beq ohai_we_are_32bit
>>> lis x,0x8000 would result in 0xffffffff80000000 or 0x80000000 in x 
>>> respectively. add. compares the full register with 0, so we also get 
>>> to analyze the shifted out 8. Very nice trick indeed and exactly 
>>> what I was searching for! Thanks a lot!
>> add. sets CR0 based on the low 32 bits of the result, in 32-bit mode. 
>>  64-bit
>> CPUs always load 0xffffffff80000000 for lis x,0x8000, no matter what 
>> mode the
>> CPU is in.
> Oh, sure, I meant that on 32-bit CPUs it won't set the f's since they 
> don't exist :)

Yeah.  And the result in 32-bit mode on a 64-bit CPU is exactly the 
same, for all
32-bit Book I instructions :-)

I don't think there is any nice way to detect the CPU you're running on 
is 64-bit
capable, when running in 32-bit mode.  But you don't need that here.


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