Re: [PATCH] powerpc: enable 64-bit mode

[Thomas Huth <thuth@redhat.com>]

On 15.03.2016 19:44, Alexander Graf wrote:
> 
> 
> On 15.03.16 19:43, Thomas Huth wrote:
>> On 15.03.2016 14:15, Laurent Vivier wrote:
>>> When they are started, processors are in 32-bit mode,
>>> as we are testing ppc64 processors, enable the 64bit mode
>>> on starting.
>>>
>>> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
>>> ---
>>>  powerpc/cstart64.S | 9 +++++++++
>>>  1 file changed, 9 insertions(+)
>>>
>>> diff --git a/powerpc/cstart64.S b/powerpc/cstart64.S
>>> index c87e3d6..634b854 100644
>>> --- a/powerpc/cstart64.S
>>> +++ b/powerpc/cstart64.S
>>> @@ -18,6 +18,15 @@
>>>  .globl start
>>>  start:
>>>  	FIXUP_ENDIAN
>>> +
>>> +	/* enable 64-bit mode */
>>> +	mfmsr	r11
>>> +	li	r12,-1
>>> +	rldicr	r12,r12,0,0
>>
>> Nice idea to set the highest bit, I didn't know that one yet :-)
>>
>>> +	or	r11,r11,r12
>>> +	mtmsrd	r11
>>> +	isync
>>
>> Looks good!
>>
>> Reviewed-by: Thomas Huth <thuth@redhat.com>
>>
>>
>> PS: As far as I can see, the C code has already been compiled for 64
>> bits ... how did that work if it has been run in 32-bit mode so far??
> 
> And why would we enter a 64bit CPU without MSR_SF set?

Ah, I see, 64-bit mode is already enabled in translate_init.c for 64-bit
CPUs! I somehow thought that 64-bit PPCs would start in 32-bit mode (for
backwards compatibility), and thus I expected that the VMs would also be
started in 32-bit mode. But seems like this is not the case... and I
learned again a new detail about PPCs :-)

 Thomas