Re: [PATCH] x86/APIC: Remove x2APIC pure cluster mode

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Sep 23, 2024 at 03:35:59PM +0100, Matthew Barnes wrote:
> With the introduction of mixed x2APIC mode (using cluster addressing for
> IPIs and physical for external interrupts) the use of pure cluster mode
> doesn't have any benefit.
> 
> Remove the mode itself, leaving only the code required for logical
> addressing when sending IPIs.
> 
> Implements: https://gitlab.com/xen-project/xen/-/issues/189

There's at least one extra bit which I would also like to see removed,
either in this patch, or as following patch.

In struct arch_irq_desc we have 3 cpumasks: cpu_mask, old_cpu_mask and
pending_mask.  After dropping cluster mode for external interrupts,
those fields could become integers AFACT, as now interrupts can only
target a single CPU opposed to a logical CPU set.

Thanks, Roger.