Re: [PATCH v2] x86/domain: adjust limitation on shared_info allocation below 4G

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

On Wed, Feb 04, 2026 at 04:32:25PM +0100, Jan Beulich wrote:
> On 04.02.2026 16:12, Andrew Cooper wrote:
> > On 04/02/2026 3:01 pm, Roger Pau Monné wrote:
> >>>> +        share_xen_page_with_guest(virt_to_page(d->shared_info), d, SHARE_rw);
> >>>> +        /* Ensure all references to the old shared_info page are dropped. */
> >>>> +        for_each_vcpu( d, v )
> >>>> +            vcpu_info_reset(v);
> >>> switch_compat() can only occur on a domain with no memory.  How can we
> >>> have outstanding references?
> >> As Jan pointed out, it's not references, but stashed pointers to the
> >> previous shared_info page.  I've used the wrong wording here.
> > 
> > Yes, I saw that thread, but my question still stands.
> > 
> > How can there be any this early in the domain's lifecycle?
> 
> Can't (aren't) vCPU-s added ahead of adding memory?

At least on x86 when using xl/libxl the call to
XEN_DOMCTL_set_address_size happens after the call to
XEN_DOMCTL_max_vcpus, and the later calls vcpu_create() which sets the
pointer into the shared_info page for legacy (< 32) vCPUs.

Even if we could invert those two calls, it's impossible to know what
other toolstacks might do.  I think we need to keep the
vcpu_info_reset() call.

Thanks, Roger.