On Wed, 13 Sep 2023, George Dunlap wrote: > On Tue, Sep 12, 2023 at 8:57 AM Thomas Gleixner <[email protected]> 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.)
Supporting CPU hotplug in a nested virtualization setting is a different proposition compared to supporting Physical CPU Hotplug. Typically virtual firmware (hypervisor-provided firmware) has less unexpected behaviors compared to baremetal firmware. Could you make the distinction in SUPPORT.md?
