Re: [PATCH v2] xenbus: Allow PVH dom0 a non-local xenstore

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 6 May 2025, Jason Andryuk wrote:
> Make xenbus_init() allow a non-local xenstore for a PVH dom0 - it is
> currently forced to XS_LOCAL.  With Hyperlaunch booting dom0 and a
> xenstore stubdom, dom0 can be handled as a regular XS_HVM following the
> late init path.
> 
> Ideally we'd drop the use of xen_initial_domain() and just check for the
> event channel instead.  However, ARM has a xen,enhanced no-xenstore
> mode, where the event channel and PFN would both be 0.  Retain the
> xen_initial_domain() check, and use that for an additional check when
> the event channel is 0.
> 
> Check the full 64bit HVM_PARAM_STORE_EVTCHN value to catch the off
> chance that high bits are set for the 32bit event channel.
> 
> Signed-off-by: Jason Andryuk <jason.andryuk@xxxxxxx>
> Change-Id: I5506da42e4c6b8e85079fefb2f193c8de17c7437
> ---
> v2:
> Re-add xen_initial_domain() check to avoid breaking ARM's xen,enhanced
> no-xenstore mode where event channel and PFN are both 0.

Thanks for the catch!

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>


>  drivers/xen/xenbus/xenbus_probe.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/xen/xenbus/xenbus_probe.c 
> b/drivers/xen/xenbus/xenbus_probe.c
> index 22d3f90ee205..b12cbd9663e3 100644
> --- a/drivers/xen/xenbus/xenbus_probe.c
> +++ b/drivers/xen/xenbus/xenbus_probe.c
> @@ -969,9 +969,15 @@ static int __init xenbus_init(void)
>       if (xen_pv_domain())
>               xen_store_domain_type = XS_PV;
>       if (xen_hvm_domain())
> +     {
>               xen_store_domain_type = XS_HVM;
> -     if (xen_hvm_domain() && xen_initial_domain())
> -             xen_store_domain_type = XS_LOCAL;
> +             err = hvm_get_parameter(HVM_PARAM_STORE_EVTCHN, &v);
> +             if (err)
> +                     goto out_error;
> +             xen_store_evtchn = (int)v;
> +             if (!v && xen_initial_domain())
> +                     xen_store_domain_type = XS_LOCAL;
> +     }
>       if (xen_pv_domain() && !xen_start_info->store_evtchn)
>               xen_store_domain_type = XS_LOCAL;
>       if (xen_pv_domain() && xen_start_info->store_evtchn)
> @@ -990,10 +996,6 @@ static int __init xenbus_init(void)
>               xen_store_interface = gfn_to_virt(xen_store_gfn);
>               break;
>       case XS_HVM:
> -             err = hvm_get_parameter(HVM_PARAM_STORE_EVTCHN, &v);
> -             if (err)
> -                     goto out_error;
> -             xen_store_evtchn = (int)v;
>               err = hvm_get_parameter(HVM_PARAM_STORE_PFN, &v);
>               if (err)
>                       goto out_error;
> -- 
> 2.34.1
>