Re: [Qemu-devel] [PATCH] s390-ccw: force diag 308 subcode to unsigned long

[Farhan Ali <alifm@linux.ibm.com>]

On 05/03/2018 11:44 AM, Cornelia Huck wrote:
> On Thu, 3 May 2018 11:25:08 -0400
> Farhan Ali <alifm@linux.ibm.com> wrote:
> 
>> On 05/02/2018 08:52 AM, Cornelia Huck wrote:
>>> We currently pass an integer as the subcode parameter. However,
>>> the upper bits of the register containing the subcode need to
>>> be 0, which is not guaranteed unless we explicitly specify the
>>> subcode to be an unsigned long value.
>>>
>>> Fixes: d046c51dad3 ("pc-bios/s390-ccw: Get device address via diag 308/6")
>>> Cc:qemu-stable@nongnu.org
>>> Signed-off-by: Cornelia Huck<cohuck@redhat.com>
>>
>> Sorry for my ignorance, but is there a C standard that says upper bits
>> of an int is not guaranteed to be 0?
> 
> The value (5 resp. 6) is small enough to fit into a regular integer,
> and the compiler generated a lhi for the load, which did not change any
> upper values that might have been in the register previously. Telling
> the compiler to treat the value as an unsigned long makes it generate a
> lghi.
> 

This makes sense now. Thanks for the explanation :)