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Re: [PATCH v2] x86/domain: adjust limitation on shared_info allocation below 4G
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Wed, 4 Feb 2026 17:54:58 +0100
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>
- Delivery-date: Wed, 04 Feb 2026 16:55:10 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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.
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