Re: [PATCH 5/5] libxc: make xc_domain_maximum_gpfn() endianness-agnostic

[Jan Beulich <jbeulich@xxxxxxxx>]

On 18.06.2021 17:24, Andrew Cooper wrote:
> On 18/06/2021 16:22, Andrew Cooper wrote:
>> On 18/06/2021 16:06, Andrew Cooper wrote:
>>> On 18/06/2021 11:25, Jan Beulich wrote:
>>>> libxc generally uses uint32_t to represent domain IDs. This is fine as
>>>> long as addresses of such variables aren't taken, to then pass into
>>>> hypercalls: To the hypervisor, a domain ID is a 16-bit value. Use an
>>>> intermediate variable to deal with the issue. (On architectures with
>>>> arguments passed in registers, such an intermediate variable would have
>>>> been created by the compiler already anyway, just one of the wrong
>>>> type.)
>>>>
>>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>>>
>>>> --- a/tools/libs/ctrl/xc_domain.c
>>>> +++ b/tools/libs/ctrl/xc_domain.c
>>>> @@ -856,7 +856,9 @@ int xc_domain_get_tsc_info(xc_interface
>>>>  
>>>>  int xc_domain_maximum_gpfn(xc_interface *xch, uint32_t domid, xen_pfn_t 
>>>> *gpfns)
>>>>  {
>>>> -    long rc = do_memory_op(xch, XENMEM_maximum_gpfn, &domid, 
>>>> sizeof(domid));
>>>> +    domid_t xen_domid = domid;
>>>> +    long rc = do_memory_op(xch, XENMEM_maximum_gpfn, &xen_domid,
>>>> +                           sizeof(xen_domid));
>>> Why on earth do we pass the domid in by pointer and not value?
>> This is horrible.
>>
>> What we're logically doing is passing a  pointer to struct
>> xen_memory_$FOO { domid_t domid; }, except its all done by void
>> pointers, and even the public header files don't provide a suitable
>> structure.
>>
>> I think we really do want to retrofit a suitable structure in the public
>> interface and use that, rather than to continue games like this.
> 
> Alternatively, declare this interface broken and reimplement it as a
> domctl, which is where the functionality ought logically to live.

Not really, no - this is something a domain can inquire on itself.

Jan