On 2018-12-18 12:45, Janosch Frank wrote:
> 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?
>
> I think I had it without that for a few days.
> What's your reasoning against it?
It just looks weird this way. Like we wanted to execute some 32-bit code
in 64-bit mode afterwards, but we don't do that.
> 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.
Fine for me, just add a comment like that ("Clear all registers to make
sure that we have a well-defined fresh start" or so).
I think I'd also rather write this as:
.irp i, 0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15
xgr \i,\i
.endr
... since that takes a little bit less space.
Thomas
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