Re: [PATCH v2 1/2] hw/xen: detect when running inside stubdomain

["Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>]

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On Tue, Mar 26, 2024 at 05:06:50PM +0000, Anthony PERARD wrote:
> On Tue, Mar 05, 2024 at 08:12:29PM +0100, Marek Marczykowski-Górecki wrote:
> > diff --git a/hw/xen/xen-legacy-backend.c b/hw/xen/xen-legacy-backend.c
> > index 124dd5f3d6..6bd4e6eb2f 100644
> > --- a/hw/xen/xen-legacy-backend.c
> > +++ b/hw/xen/xen-legacy-backend.c
> > @@ -603,6 +603,20 @@ static void xen_set_dynamic_sysbus(void)
> >      machine_class_allow_dynamic_sysbus_dev(mc, TYPE_XENSYSDEV);
> >  }
> >  
> > +static bool xen_check_stubdomain(void)
> > +{
> > +    char *dm_path = g_strdup_printf("/local/domain/%d/image", xen_domid);
> > +    int32_t dm_domid;
> > +    bool is_stubdom = false;
> > +
> > +    if (!xenstore_read_int(dm_path, "device-model-domid", &dm_domid)) {
> > +        is_stubdom = dm_domid != 0;
> > +    }
> > +
> > +    g_free(dm_path);
> > +    return is_stubdom;
> > +}
> > +
> >  void xen_be_init(void)
> >  {
> >      xenstore = qemu_xen_xs_open();
> > @@ -616,6 +630,8 @@ void xen_be_init(void)
> >          exit(1);
> >      }
> >  
> > +    xen_is_stubdomain = xen_check_stubdomain();
> 
> This isn't really a backend specific information, and xen_be_init() is
> all about old backend implementation support. (qdisk which have been the
> first to be rewritten doesn't need xen_be_init(), or shouldn't). Could
> we move the initialisation elsewhere?

I can try to move it, sure.

> Is this relevant PV guests? If not, we could move the initialisation to
> xen_hvm_init_pc().
> 
> Also, avoid having xen_check_stubdomain() depending on
> "xen-legacy-backend", if possible.
> 
> (In xen_hvm_init_pc(), a call to xen_register_ioreq() opens another
> xenstore, as `state->xenstore`.)

And xen_register_ioreq() calls xen_be_init() anyway, so it wouldn't
change much in practice (at least for now)...

> (There's already been effort to build QEMU without legacy backends, that
> stubdom check would break in this scenario.)

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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