Re: [kvm-unit-tests PATCH 3/5] powerpc: check 64bit mode

[Thomas Huth <thuth@redhat.com>]

On 18.03.2016 09:38, Paul Mackerras wrote:
> On Fri, Mar 18, 2016 at 09:28:33AM +0100, Thomas Huth wrote:
>> On 16.03.2016 16:13, Laurent Vivier wrote:
>>> Check in MSR if the SF bit is set (64bit mode is enabled)
>>>
>>> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
>>> ---
>>>  powerpc/emulator.c | 14 ++++++++++++++f958ee745f70
>>>  1 file changed, 14 insertions(+)
>>>
>>> diff --git a/powerpc/emulator.c b/powerpc/emulator.c
>>> index 1215c4f..b66c1d7 100644
>>> --- a/powerpc/emulator.c
>>> +++ b/powerpc/emulator.c
>>> @@ -32,12 +32,26 @@ static void test_illegal(void)
>>>  	report_prefix_pop();
>>>  }
>>>  
>>> +static void test_64bit(void)
>>> +{
>>> +	uint64_t msr;
>>> +
>>> +	report_prefix_push("64bit");
>>> +
>>> +	asm("mfmsr %[msr]": [msr] "=r" (msr));
>>> +
>>> +	report("detected", msr & 0x8000000000000000UL);
>>> +
>>> +	report_prefix_pop();
>>> +}
>>> +
>>>  int main(void)
>>>  {
>>>  	handle_exception(0x700, program_check_handler, (void *)&is_invalid);
>>>  
>>>  	report_prefix_push("emulator");
>>>  
>>> +	test_64bit();
>>>  	test_illegal();
>>>  
>>>  	report_prefix_pop();
>>
>> Reviewed-by: Thomas Huth <thuth@redhat.com>
>>
>> But I still wonder how kvm-unit-tests worked at all before the 64-bit
>> mode has been fixed in QEMU ... I mean kvm-unit-tests is compiled with
>> 64-bit code, but it's still working when run in 32-bit? Something really
>> strange was going on here...
> 
> On Power processors, there's not as much difference between 32-bit and
> 64-bit mode as you might expect.  In 32-bit mode on a 64-bit
> processor, you can use all 64 bits of the registers and execute 64-bit
> instructions.  The only things that change in 32-bit mode are that (a)
> effective addresses are truncated to 32 bits and (b) dot-form
> instructions (e.g. "add.") set CR0 based on the 32-bit result rather
> than the 64-bit result.

Ah, ok, ... and since kvm-unit-tests normally does not use the RAM > 4
GB, we didn't notice the truncation. Makes sense.
Thanks for the clarification!

 Thomas