Re: [PATCH v2 05/17] xen/dom0less: Workaround XSM for evtchn_alloc_unbound

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 16 Jul 2025, Jason Andryuk wrote:
> When used with an untargetable domain, xsm_evtchn_unbound fails even
> though the idle domain has is_privileged = true.  Switch current to the
> domain under construction so that event channel creation is allowed as
> XSM_TARGET.
> 
> Signed-off-by: Jason Andryuk <jason.andryuk@xxxxxxx>

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>


> ---
> v2:
> New
> 
> This was Jan's suggestion when dom0less originally had trouble with
> evtchn_alloc_unbound() and xsm_set_system_active() was introduced.  As
> noted, is_privileged is insufficient for operating on an untargetable
> domain.
> ---
>  xen/common/device-tree/dom0less-build.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/xen/common/device-tree/dom0less-build.c 
> b/xen/common/device-tree/dom0less-build.c
> index 22af043aa5..a2789e3bdf 100644
> --- a/xen/common/device-tree/dom0less-build.c
> +++ b/xen/common/device-tree/dom0less-build.c
> @@ -77,11 +77,20 @@ bool __init is_dom0less_mode(void)
>  static int __init alloc_xenstore_evtchn(struct domain *d)
>  {
>      evtchn_alloc_unbound_t alloc;
> +    struct vcpu *old_current;
>      int rc;
>  
>      alloc.dom = d->domain_id;
>      alloc.remote_dom = xs_domid;
> +    /*
> +     * Switch current from the Xen idle vcpu to the domain's vcpu.  This is 
> to
> +     * pass the xsm_evtchn_unbound() check for an untargetable domain.
> +     */
> +    old_current = current;
> +    set_current(d->vcpu[0]);
>      rc = evtchn_alloc_unbound(&alloc, 0);
> +    /* Restore Xen idle vcpu. */
> +    set_current(old_current);
>      if ( rc )
>      {
>          printk("Failed allocating event channel for domain\n");
> -- 
> 2.50.0
> 
>