Re: [kvm-unit-tests PATCH v3 04/13] s390x: Switch to z/Arch if needed

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On 18.12.18 11:04, Thomas Huth wrote:
> On 2018-12-18 10:26, Janosch Frank wrote:
>> LPAR and z/VM start in esam mode depending on the hardware, so we need
>> to switch to z/Arch and set 64 bit addressing.
>>
>> Under Qemu/KVM we already start out with both when being run with the
>> Qemu --kernel argument or we lack 64 bit when booting from disk
>> because of the initial psw specifying 31 bit for z/VM and lpar
>> compatibility.
>>
>> Signed-off-by: Janosch Frank <frankja@xxxxxxxxxxxxx>
>> Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
>> ---
>>  s390x/cstart64.S | 8 ++++++++
>>  1 file changed, 8 insertions(+)
>>
>> diff --git a/s390x/cstart64.S b/s390x/cstart64.S
>> index d382497..7886f35 100644
>> --- a/s390x/cstart64.S
>> +++ b/s390x/cstart64.S
>> @@ -20,9 +20,17 @@
>>   *
>>   * For KVM and TCG kernel boot we are in 64 bit z/Arch mode.
>>   * When booting from disk the initial short psw is in 31 bit mode.
>> + * When running under LPAR or z/VM, we might start in 31 bit and esam mode.
>>   */
>>  	.globl start
>>  start:
>> +	/* Switch to z/Architecture mode and 64-bit */
>> +	slr     %r0, %r0		# Set cpuid to zero
>> +	lhi     %r1, 2			# mode 2 = esame
>> +	sigp    %r1, %r0, 0x12		# sigp set arch
>> +	bras	%r13,0f			# Jump over zero area
>> +	.fill	16,4,0x0		# Zero area for lmh
>> +0:	lmh	%r0,%r15,0(%r13)	# Clear high-order half of gprs
> 
> Do we really want to keep the lmh here? ... yes, I know the kernel is
> doing this, too, but I really fail to see why we would need it in the
> kvm-unit-tests, too... have you tried it also without this clearing?
> 
>  Thomas
> 

I think I had it without that for a few days.
What's your reasoning against it? We loose more cycles when clearing BSS
under KVM where it's empty anyway. I'd rather expand it to a lm, to
really have a fresh start.

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