[OpenBIOS] [PATCH] ppc: fix mtmsr for SF setting
agraf at csgraf.de
Fri Jun 17 01:18:04 CEST 2011
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 :)
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