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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

~Andrew