Re: [PATCH v2] x86/x2apic: introduce a mixed physical/cluster mode

[Jan Beulich <jbeulich@xxxxxxxx>]

On 31.10.2023 15:52, Roger Pau Monne wrote:
> --- a/xen/arch/x86/genapic/x2apic.c
> +++ b/xen/arch/x86/genapic/x2apic.c
> @@ -180,6 +180,29 @@ static const struct genapic __initconstrel 
> apic_x2apic_cluster = {
>      .send_IPI_self = send_IPI_self_x2apic
>  };
>  
> +/*
> + * Mixed x2APIC mode: use physical for external (device) interrupts, and
> + * cluster for inter processor interrupts.  Such mode has the benefits of not
> + * sharing the vector space with all CPUs on the cluster, while still 
> allowing
> + * IPIs to be more efficiently delivered by not having to perform an ICR 
> write
> + * for each target CPU.
> + */
> +static const struct genapic __initconstrel apic_x2apic_mixed = {
> +    APIC_INIT("x2apic_mixed", NULL),
> +    /*
> +     * NB: IPIs use the send_IPI_{mask,self} hooks only, other fields are
> +     * exclusively used by external interrupts and hence are set to use
> +     * Physical destination mode handlers.
> +     */
> +    .int_delivery_mode = dest_Fixed,
> +    .int_dest_mode = 0 /* physical delivery */,
> +    .init_apic_ldr = init_apic_ldr_x2apic_cluster,
> +    .vector_allocation_cpumask = vector_allocation_cpumask_phys,
> +    .cpu_mask_to_apicid = cpu_mask_to_apicid_phys,
> +    .send_IPI_mask = send_IPI_mask_x2apic_cluster,
> +    .send_IPI_self = send_IPI_self_x2apic
> +};

I'm afraid the comment is still misleading in one respect: The .init_apic_ldr
hook is also set to its Clustered mode handler (and validly so). As before my
suggestion would be to leverage that we're using dedicated initializers here
and have a Physical mode portion and a Clustered mode one, each clarifying in
a brief leading comment where/how the handlers are used.

Jan