Re: [PATCH v6 5/7] xen: add new domctl get_changed_domain

[Jan Beulich <jbeulich@xxxxxxxx>]

On 07.01.2025 11:17, Juergen Gross wrote:
> --- a/xen/common/domain.c
> +++ b/xen/common/domain.c
> @@ -185,6 +185,76 @@ static void domain_changed_state(const struct domain *d)
>      unlock_dom_exc_handler(hdl);
>  }
>  
> +static void set_domain_state_info(struct xen_domctl_get_domain_state *info,
> +                                  const struct domain *d)
> +{
> +    info->state = XEN_DOMCTL_GETDOMSTATE_STATE_EXIST;
> +    if ( d->is_shut_down )
> +        info->state |= XEN_DOMCTL_GETDOMSTATE_STATE_SHUTDOWN;
> +    if ( d->is_dying == DOMDYING_dying )
> +        info->state |= XEN_DOMCTL_GETDOMSTATE_STATE_DYING;
> +    if ( d->is_dying == DOMDYING_dead )
> +        info->state |= XEN_DOMCTL_GETDOMSTATE_STATE_DEAD;
> +    info->unique_id = d->unique_id;
> +}
> +
> +int get_domain_state(struct xen_domctl_get_domain_state *info, struct domain 
> *d,
> +                     domid_t *domid)
> +{
> +    unsigned int dom;
> +    int rc = -ENOENT;
> +    struct domain *hdl;
> +
> +    if ( info->pad0 || info->pad1 )
> +        return -EINVAL;
> +
> +    if ( d )
> +    {
> +        set_domain_state_info(info, d);
> +
> +        return 0;
> +    }
> +
> +    /*
> +     * Only domain registered for VIRQ_DOM_EXC event is allowed to query
> +     * domains having changed state.
> +     */
> +    if ( !domain_handles_global_virq(current->domain, VIRQ_DOM_EXC) )
> +        return -EACCES;
> +
> +    hdl = lock_dom_exc_handler();

Instead of leaving a small window for races between the if() and this
function call, can't you simply compare hdl against current->domain?

> +    while ( dom_state_changed )
> +    {
> +        dom = find_first_bit(dom_state_changed, DOMID_MASK + 1);
> +        if ( dom >= DOMID_FIRST_RESERVED )
> +            break;
> +        if ( test_and_clear_bit(dom, dom_state_changed) )

As this is now running under lock, does it really need to be test-and-clear?
What mechanism would allow the flag to be cleared between the find-1st and
here? Plus, like for patch 4, I think it could be __clear_bit() here.

Jan