Re: [PATCH for-4.22 v3] xen/x86: Change stub page allocation/free

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

On Fri, Jun 12, 2026 at 10:35:59AM +0100, Andrew Cooper wrote:
> On 12/06/2026 9:22 am, Roger Pau Monne wrote:
> > diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
> > index 4192edf635b6..0253d22c349d 100644
> > --- a/xen/arch/x86/setup.c
> > +++ b/xen/arch/x86/setup.c
> > @@ -2089,9 +2089,7 @@ void asmlinkage __init noreturn __start_xen(void)
> >  
> >      init_idle_domain();
> >  
> > -    this_cpu(stubs.addr) = alloc_stub_page(smp_processor_id(),
> > -                                           &this_cpu(stubs).mfn);
> > -    BUG_ON(!this_cpu(stubs.addr));
> > +    init_stub();
> 
> While I hate to nitpick further, "stubs".  There are 4 per CPU, and I'm
> reasonably sure we're consistently plural elsewhere.

Funny you ask - I had it as "stubs" initially, but then realized the
existing function is named alloc_stub_page(), and hence switched back
to use "stub" as that seemed more inline with the existing usage.

I will adjust on commit.

> Otherwise (and with Jan's grammar note in the commit message too),
> Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

Thanks, yes, that's been adjusted in the commit message.

> One further thought, which would be for a followup patch if we decide to
> do it.
> 
> We do now use UDB for bugframes.  It's also marginally better than INT3
> because if we do ever end up finding ourselves executing it, because
> we'll #UD on the first one, rather than try to brute force our way
> through the #BPs printing a log message per instruction.
> 
> One thing I didn't check and I really need to is whether UDB has the
> same speculation-halting properties that UD2 has.  It's not a guarantee,
> given that the encoding is valid in other modes.

Hm, yes, let's see if I find some time to change it, or whether
someone else beats me to it :).

Roger.