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Re: [PATCH for-4.22 v3] xen/x86: Change stub page allocation/free
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Fri, 12 Jun 2026 11:51:34 +0200
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- Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx, Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>, Jason Andryuk <jason.andryuk@xxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Teddy Astie <teddy.astie@xxxxxxxxxx>, Oleksii Kurochko <oleskii.kurochko@xxxxxxxxx>
- Delivery-date: Fri, 12 Jun 2026 09:51:51 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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@xxxxxxxxxx>
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.
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