From mboxrd@z Thu Jan  1 00:00:00 1970
From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Subject: Re: [PATCH v3 2/6] libxl: do not add a vkb backend to
 hvm guests
Date: Thu, 2 Jul 2015 10:31:31 -0400
Message-ID: <20150702143130.GA4513@l.oracle.com>
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> > But I am looking at the xen-fbfront.c driver and it might be that
> > I had already fixed this issue! (inadvertly it seems)
> > 
> > 51c71a3bbaca868043cc45b3ad3786dd48a90235
> > Author: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> > Date:   Tue Nov 26 15:05:40 2013 -0500
> > 
> >     xen/pvhvm: If xen_platform_pci=0 is set don't blow up (v4).
> > 
> > ..
> >    - if running in HVM, check if user wanted 'xen_emul_unplug=never',
> >        in which case bail out and don't load any PV drivers.
> >      - if running in HVM, and if PCI device 5853:0001 (xen_platform_pci)
> >        does not exist, then bail out and not load PV drivers.
> >      - (v2) if running in HVM, and if the user wanted 'xen_emul_unplug=ide-disks',
> >        then bail out for all PV devices _except_ the block one.
> >        Ditto for the network one ('nics').
> >      - (v2) if running in HVM, and if the user wanted 'xen_emul_unplug=unnecessary'
> >        then load block PV driver, and also setup the legacy IDE paths.
> >        In (v3) make it actually load PV drivers.
> > 
> > .. which means that if the driver does not use the 'xen_has_pv_XXX_devices'
> > but only the 'xen_has_pv_devices' then for a normal HVM guest it won't load
> > it.
> > 
> > And sure enough we have:
> > 
> > +       if (!xen_has_pv_devices())
> > +               return -ENODEV;
> > 
> > so we bail out and not load it under HVM.
>  
> And at the same time it works on ARM because CONFIG_XEN_PVHVM is not
> defined there, right?

Yup, and it ends up doing:

static inline bool xen_has_pv_devices(void)                                     
{                                                                               
        return IS_ENABLED(CONFIG_XEN);                                          
}                      

which will return true if CONFIG_XEN is set.