x86/APIC: Remove x2APIC pure cluster mode

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.

Resolves: https://gitlab.com/xen-project/xen/-/issues/189
Signed-off-by: Matthew Barnes <matthew.barnes@cloud.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>

CHANGELOG.md

@@ -16,6 +16,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
  - On x86:
    - Support for running on Xeon Phi processors.
    - Removed the `ucode=allow-same` command line option.
+   - Removed x2APIC Cluster Mode for external interrupts.  x2APIC Physical and
+     Mixed Modes are still available.
 
 ## [4.19.0](https://xenbits.xenproject.org/gitweb/?p=xen.git;a=shortlog;h=RELEASE-4.19.0) - 2024-07-29
 

docs/misc/xen-command-line.pandoc

@@ -2842,10 +2842,10 @@ the watchdog.
 Permit use of x2apic setup for SMP environments.
 
 ### x2apic-mode (x86)
-> `= physical | cluster | mixed`
+> `= physical | mixed`
 
 > Default: `physical` if **FADT** mandates physical mode, otherwise set at
->          build time by CONFIG_X2APIC_{PHYSICAL,LOGICAL,MIXED}.
+>          build time by CONFIG_X2APIC_{PHYSICAL,MIXED}.
 
 In the case that x2apic is in use, this option switches between modes to
 address APICs in the system as interrupt destinations.

xen/arch/x86/Kconfig

@@ -275,18 +275,6 @@ config X2APIC_PHYSICAL
 	  destination inter processor interrupts (IPIs) slightly slower than
 	  Logical Destination mode.
 
-config X2APIC_CLUSTER
-	bool "Cluster Destination mode"
-	help
-	  When using this mode APICs are addressed using the Cluster Logical
-	  Destination mode.
-
-	  Cluster Destination has the benefit of sending IPIs faster since
-	  multiple APICs can be targeted as destinations of a single IPI.
-	  However the vector space is shared between all CPUs on the cluster,
-	  and hence using this mode reduces the number of available vectors
-	  when compared to Physical mode.
-
 config X2APIC_MIXED
 	bool "Mixed Destination mode"
 	help

xen/arch/x86/genapic/x2apic.c

@@ -63,26 +63,6 @@ static void cf_check init_apic_ldr_x2apic_cluster(void)
     cpumask_set_cpu(this_cpu, per_cpu(cluster_cpus, this_cpu));
 }
 
-static const cpumask_t *cf_check vector_allocation_cpumask_x2apic_cluster(
-    int cpu)
-{
-    return per_cpu(cluster_cpus, cpu);
-}
-
-static unsigned int cf_check cpu_mask_to_apicid_x2apic_cluster(
-    const cpumask_t *cpumask)
-{
-    unsigned int cpu = cpumask_any(cpumask);
-    unsigned int dest = per_cpu(cpu_2_logical_apicid, cpu);
-    const cpumask_t *cluster_cpus = per_cpu(cluster_cpus, cpu);
-
-    for_each_cpu ( cpu, cluster_cpus )
-        if ( cpumask_test_cpu(cpu, cpumask) )
-            dest |= per_cpu(cpu_2_logical_apicid, cpu);
-
-    return dest;
-}
-
 static void cf_check send_IPI_self_x2apic(uint8_t vector)
 {
     apic_wrmsr(APIC_SELF_IPI, vector);
@@ -169,17 +149,6 @@ static const struct genapic __initconst_cf_clobber apic_x2apic_phys = {
     .send_IPI_self = send_IPI_self_x2apic
 };
 
-static const struct genapic __initconst_cf_clobber apic_x2apic_cluster = {
-    APIC_INIT("x2apic_cluster", NULL),
-    .int_delivery_mode = dest_LowestPrio,
-    .int_dest_mode = 1 /* logical delivery */,
-    .init_apic_ldr = init_apic_ldr_x2apic_cluster,
-    .vector_allocation_cpumask = vector_allocation_cpumask_x2apic_cluster,
-    .cpu_mask_to_apicid = cpu_mask_to_apicid_x2apic_cluster,
-    .send_IPI_mask = send_IPI_mask_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
@@ -252,15 +221,13 @@ static int8_t __initdata x2apic_phys = -1;
 boolean_param("x2apic_phys", x2apic_phys);
 
 enum {
-   unset, physical, cluster, mixed
+   unset, physical, mixed
 } static __initdata x2apic_mode = unset;
 
 static int __init cf_check parse_x2apic_mode(const char *s)
 {
     if ( !cmdline_strcmp(s, "physical") )
         x2apic_mode = physical;
-    else if ( !cmdline_strcmp(s, "cluster") )
-        x2apic_mode = cluster;
     else if ( !cmdline_strcmp(s, "mixed") )
         x2apic_mode = mixed;
     else
@@ -274,7 +241,7 @@ const struct genapic *__init apic_x2apic_probe(void)
 {
     /* Honour the legacy cmdline setting if it's the only one provided. */
     if ( x2apic_mode == unset && x2apic_phys >= 0 )
-        x2apic_mode = x2apic_phys ? physical : cluster;
+        x2apic_mode = x2apic_phys ? physical : mixed;
 
     if ( x2apic_mode == unset )
     {
@@ -286,21 +253,12 @@ const struct genapic *__init apic_x2apic_probe(void)
         else
             x2apic_mode = IS_ENABLED(CONFIG_X2APIC_MIXED) ? mixed
                           : (IS_ENABLED(CONFIG_X2APIC_PHYSICAL) ? physical
-                                                                : cluster);
+                                                                : mixed);
     }
 
     if ( x2apic_mode == physical )
         return &apic_x2apic_phys;
 
-    if ( x2apic_mode == cluster && iommu_intremap != iommu_intremap_full )
-    {
-        printk("WARNING: x2APIC cluster mode is not supported %s interrupt remapping -"
-               " forcing mixed mode\n",
-               iommu_intremap == iommu_intremap_off ? "without"
-                                                    : "with restricted");
-        x2apic_mode = mixed;
-    }
-
     if ( !this_cpu(cluster_cpus) )
     {
         update_clusterinfo(NULL, CPU_UP_PREPARE,
@@ -309,7 +267,7 @@ const struct genapic *__init apic_x2apic_probe(void)
         register_cpu_notifier(&x2apic_cpu_nfb);
     }
 
-    return x2apic_mode == cluster ? &apic_x2apic_cluster : &apic_x2apic_mixed;
+    return &apic_x2apic_mixed;
 }
 
 void __init check_x2apic_preenabled(void)