Re: [PATCH v3 2/2] xen: introduce CONFIG_HAS_SHARED_INFO for archs without a shared page

[Jan Beulich <jbeulich@xxxxxxxx>]

On 25.06.2026 13:19, Oleksii Kurochko wrote:
> 
> 
> On 6/25/26 1:04 PM, Jan Beulich wrote:
>> On 25.06.2026 12:50, Oleksii Kurochko wrote:
>>> On 6/17/26 3:26 PM, Jan Beulich wrote:
>>>>> +#define shared_info(d, field) \
>>>>> +    (*(typeof(__shared_info(d, (d)->shared_info, field)) 
>>>>> *)shared_info_absent())
>>>> How about the simpler
>>>>
>>>> extern struct shared_info *shared_info_absent;
>>>> #define shared_info(d, field) (shared_info_absent->field)
>>>>
>>>> ?
>>>
>>> This could lead to compilation error:
>>>
>>> common/domain.c: In function 'vcpu_info_reset':
>>> common/domain.c:316:20: error: unused variable 'd' [-Werror=unused-variable]
>>>     316 |     struct domain *d = v->domain;
>>>         |                    ^
>>> cc1: all warnings being treated as errors
>>
>> #define shared_info(d, field) ((void)(d), shared_info_absent->field)
>>
>> Which it should have been anyway, to make sure the first argument is 
>> evaluated.
> 
> I expected that as shared_info() is used as an lvalue whose address is 
> taken (&shared_info(d, vcpu_info[...])). To "consume" d we can't just 
> prepend a comma expression as &(comma_expr) is illegal since a comma 
> result isn't an lvalue.
> 
> Trying what you suggested leads to:
> common/domain.c: In function 'vcpu_info_reset':
> common/domain.c:320:26: error: lvalue required as unary '&' operand
>    320 |         ? (vcpu_info_t *)&shared_info(d, vcpu_info[v->vcpu_id])

Ah yes.

> Thereby this option:
>    #define shared_info(d, field) (*((void)(d), &shared_info_absent->field))
> will be better.

Or maybe

#define shared_info(d, field) (((void)(d), shared_info_absent)->field)

?

Jan