Re: [XEN PATCH] tools: add x2APIC entries in MADT based on APIC ID

[Matthew Barnes <matthew.barnes@xxxxxxxxx>]

On Tue, Mar 26, 2024 at 05:15:46PM +0100, Jan Beulich wrote:
> On 26.03.2024 16:57, Matthew Barnes wrote:
> >>> This patch scans each APIC ID before constructing the MADT, and uses the
> >>> x2APIC entry for each vCPU whose APIC ID exceeds the size limit imposed
> >>> by regular APIC entries.
> >>
> >> It is my understanding that if you use any x2APIC entry, every CPU needs
> >> to have one.
> > 
> > In the ACPI 6.4 specification, section 5.2.12.12, the note says the 
> > following:
> > 
> > [Compatibility note] On some legacy OSes, Logical processors with APIC ID
> > values less than 255 (whether in XAPIC or X2APIC mode) must use the 
> > Processor
> > Local APIC structure to convey their APIC information to OSPM, and those
> > processors must be declared in the DSDT using the Processor() keyword.
> > 
> > Therefore, even in X2APIC mode, it's better to represent processors with 
> > APIC
> > ID values less than 255 with APIC entries for legacy reasons.
> 
> Well, my reading of that is different: All CPUs need to have x2APIC entries
> if one has, and CPUs with small enough APIC IDs _additionally_ need xAPIC
> entries. That's what I've been observing on real hardware, too.

Understood. Patch v2 will reflect this

Matt