Re: [RFC PATCH v4 4/7] xen/pvh: Move Xen specific PVH VM initialization out of common code

[Maran Wilson]

On 3/1/2018 8:05 AM, Boris Ostrovsky wrote:
> On 02/28/2018 01:28 PM, Maran Wilson wrote:
> > We need to refactor PVH entry code so that support for other hypervisors
> > like Qemu/KVM can be added more easily.
> >
> > This patch moves the small block of code used for initializing Xen PVH
> > virtual machines into the Xen specific file. This initialization is not
> > going to be needed for Qemu/KVM guests. Moving it out of the common file
> > is going to allow us to compile kernels in the future without CONFIG_XEN
> > that are still capable of being booted as a Qemu/KVM guest via the PVH
> > entry point.
> >
> > Signed-off-by: Maran Wilson <maran.wilson@xxxxxxxxxx>
> > ---
> >   arch/x86/pvh.c               | 28 ++++++++++++++++++++--------
> >   arch/x86/xen/enlighten_pvh.c | 18 +++++++++++++++++-
> >   2 files changed, 37 insertions(+), 9 deletions(-)
> >
> > diff --git a/arch/x86/pvh.c b/arch/x86/pvh.c
> > index b56cb5e7d6ac..2d7a7f4958cb 100644
> > --- a/arch/x86/pvh.c
> > +++ b/arch/x86/pvh.c
> > @@ -72,26 +72,38 @@ static void __init init_pvh_bootparams(void)
> >   	pvh_bootparams.hdr.type_of_loader = (9 << 4) | 0; /* Xen loader */
> >   }
> >   
> > +/*
> > + * If we are trying to boot a Xen PVH guest, it is expected that the kernel
> > + * will have been configured to provide the required override for this routine.
> > + */
> > +void __init __weak xen_pvh_init(void)
> > +{
> > +	xen_raw_printk("Error: Missing xen PVH initialization\n");
> I think this should be printk (or, more precisely, this should not be
> xen_raw_printk()): we are here because we are *not* a Xen guest and so
> Xen-specific printk will not work. (and the same is true for the next
> patch where weak mem_map_via_hcall() is added).

Actually I left that xen_raw_printk() statement in on purpose. It's also 
possible that some future developer accidentally drops or hides the 
strong version of the routine even when CONFIG_XEN (and CONFIG_HVC_XEN) 
is enabled. In that situation, this error message might prove helpful in 
quickly identifying the problem when he or she attempts to boot a Xen guest.

And in situations where CONFIG_XEN is disabled or someone is booting a 
non xen guest, the statement simply becomes a nop, so no harm is done. 
And also, I believe this code is far too early for normal printk() 
statements to work so switching to that won't buy us anything.

Thanks,
-Maran

> -boris
>
>
> > +	BUG();
> > +}