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Re: hardware domain and control domain separation
- To: Stefano Stabellini <sstabellini@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 23 Jun 2025 17:44:28 +0200
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- Cc: jason.andryuk@xxxxxxx, Julien Grall <julien@xxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, ayankuma@xxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Mon, 23 Jun 2025 15:44:58 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 21.06.2025 02:41, Stefano Stabellini wrote:
> Regarding hardware domain and control domain separation, Ayan sent to
> xen-devel an architecture specification (a design document) that I wrote
> previously about the topic. This is written as safety document so it is
> using a language and structure specific for that. However, it contains
> much of the explanation needed on the topic:
>
> https://lore.kernel.org/xen-devel/20250304183115.2509666-1-ayan.kumar.halder@xxxxxxx/
Yet even there the line between Hardware and Control is already blurred
imo. Take "Reboot and shutdown the platform", for example. It seems
pretty likely that Hardware has ways to achieve that without involving
a hypercall. You can also see that (kind of) connection in the
hypervisor itself: The special handling of a domain shutting down is
in hwdom_shutdown(), with the call to it keyed to is_hardware_domain()
(as is to be expected from the function's name).
Furthermore there it is (again) assumed that Control has full privileges.
I did mention before that I'm not convinced any domain, in a
disaggregated setup, would need to have (nor should have) full privilege.
Also a more fundamental question I was wondering about: If Control had
full privilege, nothing else in the system ought to be able to interfere
with it. Yet then how does that domain communicate with the outside
world? It can't have PV or Virtio drivers after all. And even if its
sole communication channel was a UART, Hardware would likely be able to
interfere.
Jan
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