Re: [PATCH] x86/hvm: short-circuit HVM shadow guest creation earlier

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Feb 09, 2026 at 05:02:26PM +0100, Alejandro Vallejo wrote:
> On Mon Feb 9, 2026 at 3:40 PM CET, Roger Pau Monne wrote:
> > If shadow paging has been compiled out short circuit the creation of HVM
> > guests that attempt to use shadow paging at arch_sanitise_domain_config().
> > There's no need to further build the domain when creation is doomed to fail
> > later on.
> >
> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > ---
> >  xen/arch/x86/domain.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
> > index 8b2f33f1a06c..8eb1509782ef 100644
> > --- a/xen/arch/x86/domain.c
> > +++ b/xen/arch/x86/domain.c
> > @@ -627,6 +627,12 @@ int arch_sanitise_domain_config(struct 
> > xen_domctl_createdomain *config)
> >          return -EINVAL;
> >      }
> >  
> > +    if ( hvm && !hap && !IS_ENABLED(CONFIG_SHADOW_PAGING) )
> > +    {
> > +        dprintk(XENLOG_INFO, "Shadow paging requested but not 
> > available\n");
> 
> nit: s/requested/required/, maybe?

The wording matches the rest of the messages in
arch_sanitise_domain_config().  I'm not saying that makes it correct,
but if we word this differently we should also change the others
IMO.

> Also, with this in place can't we get rid of the panic in create_dom0() that
> checks an identical condition?

Hm, I would possibly leave that one, as I think it's clearer for the
dom0 case.  Otherwise someone using a build without HAP or shadow and
attempting to boot in PVH mode will get a message saying: "Shadow
paging requested but not available", which is IMO less clear than
getting a "Neither HAP nor Shadow available for PVH domain" error
message.

Just my thinking, both checks achieve the same result, but the error
message in the create_dom0() instance is more helpful in the context
of dom0 creation.

Thanks, Roger.