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

[Segher Boessenkool]

>>> 	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 :-)

>> 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.


Segher