Re: [PATCH v2] xen/x86: On x2APIC mode, derive LDR from APIC_ID

[Alejandro Vallejo <alejandro.vallejo@cloud.com>]

On Tue, Nov 21, 2023 at 04:26:04PM +0000, Alejandro Vallejo wrote:
> Both Intel and AMD manuals agree that on x2APIC mode, the APIC LDR and ID
> registers are derivable from each other through a fixed formula.
> 
> Xen uses that formula, but applies it to vCPU IDs (which are sequential)
> rather than x2APIC_IDs (which are not, at the moment). As I understand it,
> this is an attempt to tightly pack vCPUs into clusters so each cluster has
> 16 vCPUs rather than 8, but this is problematic for OSs that might read the
> x2APIC_ID and internally derive LDR (or the other way around)


Ugh, forgot about Roger's commit message request
>I would replace the underscore from x2APIC ID with a space instead.
Happy for that to happen on commit if the rest looks ok.

Cheers,
Alejandro