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

[Alexander Graf]

On 17.06.2011, at 00:56, Segher Boessenkool wrote:

>>>> 	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.

>>> 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 :)


Alex