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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 7 Sep 2023, Jan Beulich wrote:
> On 06.09.2023 22:49, Stefano Stabellini wrote:
> > On Fri, 1 Sep 2023, Jan Beulich wrote:
> >> On 07.08.2023 11:38, Simon Gaiser wrote:
> >>> It seems some firmwares put dummy entries in the ACPI MADT table for non
> >>> existing processors. On my NUC11TNHi5 those have the invalid APIC ID
> >>> 0xff. Linux already has code to handle those cases both in
> >>> acpi_parse_lapic [1] as well as in acpi_parse_x2apic [2]. So add the
> >>> same check to Xen.
> >>>
> >>> Note that on some older (2nd gen Core i) laptop of mine I also saw dummy
> >>> entries with a valid APIC ID. Linux would still ignore those because
> >>> they have !ACPI_MADT_ENABLED && !ACPI_MADT_ONLINE_CAPABLE. But in Xen
> >>> this check is only active for madt_revision >= 5. But since this version
> >>> check seems to be intentionally I leave that alone.
> >>>
> >>> Link: 
> >>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f3bf1dbe64b62a2058dd1944c00990df203e8e7a
> >>>  # [1]
> >>> Link: 
> >>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=10daf10ab154e31237a8c07242be3063fb6a9bf4
> >>>  # [2]
> >>> Signed-off-by: Simon Gaiser <simon@xxxxxxxxxxxxxxxxxxxxxx>
> >>
> >> This patch was committed with unaddressed review comments. The normal 
> >> action
> >> in such a case would be to revert, expecting a v2 to arrive. One 
> >> alternative
> >> here would be a timely incremental patch submission. Another alternative,
> >> considering in particular Thomas's most recent reply, would be to properly
> >> downgrade CPU hotplug support in SUPPORT.md (with a corresponding entry in
> >> CHANGELOG.md).
> > 
> > I am in favor of downgrading physical CPU hotplug support in
> > SUPPORT.md.
> > 
> > I noticed that there is no entry for physical CPU hotplug support in
> > SUPPORT.md today. Should we assume that it is not supported already as
> > it is not listed as supported?
> 
> Hmm, I see
> 
> ## Host hardware support
> 
> ### Physical CPU Hotplug
> 
>     Status, x86: Supported
> 
> pretty close to the top of the file.

Ops, it must have been the case-sensitive search that failed me


> > Specifically, would it be a good idea to add a sentence at the top of
> > the file saying that anything not explicitly listed is not supported?
> 
> Iirc that was the plan to do for 4.18, but then we need to be sure that
> things don't unintentionally become unsupported. I've no clear idea how
> this plan was meant to be carried out, though.

it would be interesting to discuss it again