Re: [XEN PATCH] x86/ACPI: Ignore entries with invalid APIC IDs when parsing MADT

[George Dunlap <george.dunlap@xxxxxxxxx>]

On Tue, Sep 12, 2023 at 8:57 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>
> On Mon, Sep 11 2023 at 19:24, Andrew Cooper wrote:
> > Furthermore, cursory testing that Thomas did for the Linux topology work
> > demonstrates that it is broken anyway for reasons unrelated to ACPI parsing.
> >
> > Even furthermore, it's an area of the Xen / dom0 boundary which is
> > fundamentally broken for non-PV cases, and undocumented for the PV case,
> > hence why it's broken in Linux.
> >
> > Physical CPU Hotplug does not pass the bar for being anything more than
> > experimental.  It's absolutely not tech-preview level because the only
> > demo it has had in an environment (admittedly virtual) which does
> > implement the spec in a usable way demonstrates that it doesn't function.
> >
> > The fact no-one has noticed until now shows that the feature isn't used,
> > which comes back around full circle to the fact that Intel never made it
> > work and never shipped it.
>
> OTOH it _is_ used in virtualization. KVM supports it and it just
> works. That's how I found out that XEN explodes in colourful ways :)

It should be pointed out that there's currently a start-up selling a
product that specifically runs Xen under cloud providers -- Exostellar
(was Exotanium) [1].  If cloud providers do use ACPI hotplug to allow
on-the-fly adjustments of the number of vcpus, Exostellar will
probably want support at some point.  (Perhaps it would be good to
rope them into testing / maintaining it.)

 -George

[1] https://exostellar.io/