Re: [kvm-unit-tests PATCH v3 1/1] s390x: verify EQBS/SQBS is unavailable

[Janosch Frank <frankja@linux.ibm.com>]

On 8/4/22 00:17, Thomas Huth wrote:
> On 03/08/2022 15.58, Nico Boehr wrote:
>> QEMU doesn't provide EQBS/SQBS instructions, so we should check they
>> result in an exception.
> 
> I somewhat fail to see the exact purpose of this patch... QEMU still doesn't
> emulate a lot of other instructions, too, so why are we checking now these
> QBS instructions? Why not all the others? Why do we need a test to verify
> that there is an exception in this case - was there a bug somewhere that
> didn't cause an exception in certain circumstances?

Looking at the patch that introduced the QEMU handlers (1eecf41b) I 
wonder why those two cases were added. From my point of view it makes 
sense to remove the special handling for those two instructions.

@Christian: Any idea why this was added? Can we remove it?

The only reason I can think of to test this is the existence of EC* bits 
that control the behavior for those instructions. So if we set those 
without having QEMU handling code then we're in trouble.

But then I'd also expect that we need to set a stfle bit to indicate the 
availability and this test doesn't check for that and would indicate a 
false-positive.