Re: [PATCH kvm-unit-tests 1/2] s390x: diag288: Add missing clobber

[Thomas Huth <thuth@redhat.com>]

On 17/12/2021 15.16, Janosch Frank wrote:
> On 12/17/21 14:47, Thomas Huth wrote:
>> On 17/12/2021 11.31, Nico Boehr wrote:
>>> We clobber r0 and thus should let the compiler know we're doing so.
>>>
>>> Because we change from basic to extended ASM, we need to change the
>>> register names, as %r0 will be interpreted as a token in the assembler
>>> template.
>>>
>>> For consistency, we align with the common style in kvm-unit-tests which
>>> is just 0.
>>>
>>> Signed-off-by: Nico Boehr <nrb@linux.ibm.com>
>>> ---
>>>    s390x/diag288.c | 7 ++++---
>>>    1 file changed, 4 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/s390x/diag288.c b/s390x/diag288.c
>>> index 072c04a5cbd6..da7b06c365bf 100644
>>> --- a/s390x/diag288.c
>>> +++ b/s390x/diag288.c
>>> @@ -94,11 +94,12 @@ static void test_bite(void)
>>>        /* Arm watchdog */
>>>        lc->restart_new_psw.mask = extract_psw_mask() & ~PSW_MASK_EXT;
>>>        diag288(CODE_INIT, 15, ACTION_RESTART);
>>> -    asm volatile("        larl    %r0, 1f\n"
>>> -             "        stg    %r0, 424\n"
>>> +    asm volatile("        larl    0, 1f\n"
>>> +             "        stg    0, 424\n"
>>
>> Would it work to use %%r0 instead?
> 
> Yes, but I told him that looks weird, so that one is on me.
> @claudio @thomas What's your preferred way of dealing with this?

I think it's mostly a matter of taste. I slightly prefer %%r0 to just 0 so 
that it is clear from the first sight that it is a register and not an 
immediate constant.
Additionally, there used to be a problem with older versions of Clang that 
required the %%rX syntax, see:

  https://git.qemu.org/?p=qemu.git;a=commitdiff;h=052b66e7211af6

But we're not supporting those Clang versions for the kvm-unit-tests anyway, 
so that doesn't really count.

Thus, I don't mind too much, if everybody else prefers the bare numbers, 
then let's go with this.

  Thomas