On 16.06.2011, at 21:43, Segher Boessenkool wrote:
adds a runtime check on PVR to see if we're running on a 64-bit
capable CPU. If so, we use mtmsrd, which can set the high 32bits of MSR.
That PVR check isn't quite correct.
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
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,
Instead of the PVR thing, you could check if MSR[SF]
is set, and only then
clear it. Is this same code used on 32-bit systems?
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!