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

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

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])

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

~ Oleksii