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

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

On Tue, Sep 24, 2024 at 06:21:47PM +0200, Jan Beulich wrote:
> On 24.09.2024 18:14, Roger Pau Monné wrote:
> > On Tue, Sep 24, 2024 at 04:27:36PM +0100, Andrew Cooper wrote:
> >> On 24/09/2024 4:10 pm, Roger Pau Monné wrote:
> >>> 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
> >>
> >> We use the Resolves: tag for this.  Can fix on commit.
> >>
> >>> 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.
> >>
> >> A separate patch for sure, but that sounds like a great improvement.
> > 
> > Oh, there are quite some fields of struct genapic that are not needed
> > anymore, since both implementations set it to the same function.  It
> > would be helpful to prune them.
> 
> Pruning where possible - yes. But "both" won't cover it, as we have 4
> instances of the struct (not just the two x2apic ones).

Yeah, realized that afterwards, we still have the xAPIC flat mode,
which is using logical delivery mode, but target a single CPU.  So
getting rid of the cpumask in arch_irq_desc seem possible, however
there might be nothing to prune in struct genapic.

Regards, Roger.