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

[Andrew Cooper <andrew.cooper3@citrix.com>]

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@suse.com>
>>
>> --- 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.

~Andrew