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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 03.06.2026 16:25, Oleksii Kurochko wrote:
> --- a/xen/common/domain.c
> +++ b/xen/common/domain.c
> @@ -320,9 +320,9 @@ void vcpu_info_reset(struct vcpu *v)
>      struct domain *d = v->domain;
>  
>      v->vcpu_info_area.map =
> -        ((v->vcpu_id < XEN_LEGACY_MAX_VCPUS)
> -         ? (vcpu_info_t *)&shared_info(d, vcpu_info[v->vcpu_id])
> -         : &dummy_vcpu_info);
> +        IS_ENABLED(CONFIG_HAS_SHARED_INFO) && v->vcpu_id < 
> XEN_LEGACY_MAX_VCPUS
> +        ? (vcpu_info_t *)&shared_info(d, vcpu_info[v->vcpu_id])
> +        : &dummy_vcpu_info;
>  }

While the change here is likely okay, it points at possible further omissions.
You've dealt with all uses of shared_info(), but you've left alone all uses of
vcpu_info() (and __vcpu_info()). Reads are presumably okay, but writes to
dummy_vcpu_info open a side channel for possible info leaks. Looking over uses
in common code, no code changes may be needed; extending the description may
be all that's wanted here.

> --- a/xen/common/event_channel.c
> +++ b/xen/common/event_channel.c
> @@ -1323,9 +1323,13 @@ int evtchn_reset(struct domain *d, bool resuming)
>          rc = -EAGAIN;
>      else if ( d->evtchn_fifo )
>      {
> -        /* Switching back to 2-level ABI. */
>          evtchn_fifo_destroy(d);
> -        evtchn_2l_init(d);
> +
> +        if ( IS_ENABLED(CONFIG_HAS_SHARED_INFO) )
> +            /* Switching back to 2-level ABI. */
> +            evtchn_2l_init(d);
> +        else
> +            evtchn_fifo_init_ops(d);
>      }

The "else" part isn't needed, is it? evtchn_fifo_destroy() doesn't undo
what ...

> @@ -1624,7 +1628,11 @@ void evtchn_check_pollers(struct domain *d, unsigned 
> int port)
>  
>  int evtchn_init(struct domain *d, unsigned int max_port)
>  {
> -    evtchn_2l_init(d);
> +    if ( IS_ENABLED(CONFIG_HAS_SHARED_INFO) )
> +        evtchn_2l_init(d);
> +    else
> +        evtchn_fifo_init_ops(d);

... was done here.

> --- a/xen/include/xen/shared.h
> +++ b/xen/include/xen/shared.h
> @@ -43,7 +43,14 @@ typedef struct vcpu_info vcpu_info_t;
>  
>  extern vcpu_info_t dummy_vcpu_info;
>  
> +#ifdef CONFIG_HAS_SHARED_INFO
>  #define shared_info(d, field)      __shared_info(d, (d)->shared_info, field)
> +#else
> +void * shared_info_absent(void);

Nit: Stray blank after '*'.

> +#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)

?

Jan