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

["Roger Pau Monné" <roger.pau@citrix.com>]

On Mon, Feb 09, 2026 at 05:26:54PM +0100, Alejandro Vallejo wrote:
> On Mon Feb 9, 2026 at 5:11 PM CET, Roger Pau Monné wrote:
> > 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@citrix.com>
> >> > ---
> >> >  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.
> 
> My point is rather that HAP, or relaxed MSRs, or other settings are actively
> requested via createdomain flags. Shadow is instead the consequence of not
> setting HAP. You don't request shadow, you either requested something else or
> you hit the error.
> 
> It's not terrible wording, just imprecise.

Hm, I see.  I've always considered not requesting HAP equivalent to
requesting shadow.  IOW: not setting XEN_DOMCTL_CDF_hap doesn't mean
"pick any possible option" but rather it's an explicit request to use
shadow paging on HVM.

The interface would possibly be cleaner if we had both a HAP and
Shadow flags, and lack of any being selected could mean for Xen to
pick the default one.

Thanks, Roger.