A set of changes for the x86 APIC code:

- Handle the case where x2APIC is enabled and locked by the BIOS on a
     kernel with CONFIG_X86_X2APIC=n gracefully. Instead of a panic which
     does not make it to the graphical console during very early boot,
     simply disable the local APIC completely and boot with the PIC and very
     limited functionality, which allows to diagnose the issue.
 
   - Convert x86 APIC device tree bindings to YAML
 
   - Extend x86 APIC device tree bindings to configure interrupt delivery
     mode and handle this in during init. This allows to boot with device
     tree on platforms which lack a legacy PIC.
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Merge tag 'x86-apic-2022-12-10' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 apic update from Thomas Gleixner:
 "A set of changes for the x86 APIC code:

   - Handle the case where x2APIC is enabled and locked by the BIOS on a
     kernel with CONFIG_X86_X2APIC=n gracefully.

     Instead of a panic which does not make it to the graphical console
     during very early boot, simply disable the local APIC completely
     and boot with the PIC and very limited functionality, which allows
     to diagnose the issue

   - Convert x86 APIC device tree bindings to YAML

   - Extend x86 APIC device tree bindings to configure interrupt
     delivery mode and handle this in during init. This allows to boot
     with device tree on platforms which lack a legacy PIC"

* tag 'x86-apic-2022-12-10' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/of: Add support for boot time interrupt delivery mode configuration
  x86/of: Replace printk(KERN_LVL) with pr_lvl()
  dt-bindings: x86: apic: Introduce new optional bool property for lapic
  dt-bindings: x86: apic: Convert Intel's APIC bindings to YAML schema
  x86/of: Remove unused early_init_dt_add_memory_arch()
  x86/apic: Handle no CONFIG_X86_X2APIC on systems with x2APIC enabled by BIOS

Documentation/devicetree/bindings/interrupt-controller/intel,ce4100-ioapic.txt

@@ -1,26 +0,0 @@
-Interrupt chips
----------------
-
-* Intel I/O Advanced Programmable Interrupt Controller (IO APIC)
-
-  Required properties:
-  --------------------
-     compatible = "intel,ce4100-ioapic";
-     #interrupt-cells = <2>;
-
-  Device's interrupt property:
-
-     interrupts = <P S>;
-
-  The first number (P) represents the interrupt pin which is wired to the
-  IO APIC. The second number (S) represents the sense of interrupt which
-  should be configured and can be one of:
-    0 - Edge Rising
-    1 - Level Low
-    2 - Level High
-    3 - Edge Falling
-
-* Local APIC
-  Required property:
-
-     compatible = "intel,ce4100-lapic";

Documentation/devicetree/bindings/interrupt-controller/intel,ce4100-ioapic.yaml

@@ -0,0 +1,60 @@
+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: "http://devicetree.org/schemas/interrupt-controller/intel,ce4100-ioapic.yaml#"
+$schema: "http://devicetree.org/meta-schemas/core.yaml#"
+
+title: Intel I/O Advanced Programmable Interrupt Controller (IO APIC)
+
+maintainers:
+  - Rahul Tanwar <rtanwar@maxlinear.com>
+
+description: |
+  Intel's Advanced Programmable Interrupt Controller (APIC) is a
+  family of interrupt controllers. The APIC is a split
+  architecture design, with a local component (LAPIC) integrated
+  into the processor itself and an external I/O APIC. Local APIC
+  (lapic) receives interrupts from the processor's interrupt pins,
+  from internal sources and from an external I/O APIC (ioapic).
+  And it sends these to the processor core for handling.
+  See [1] Chapter 8 for more details.
+
+  Many of the Intel's generic devices like hpet, ioapic, lapic have
+  the ce4100 name in their compatible property names because they
+  first appeared in CE4100 SoC.
+
+  This schema defines bindings for I/O APIC interrupt controller.
+
+  [1] https://pdos.csail.mit.edu/6.828/2008/readings/ia32/IA32-3A.pdf
+
+properties:
+  compatible:
+    const: intel,ce4100-ioapic
+
+  reg:
+    maxItems: 1
+
+  interrupt-controller: true
+
+  '#interrupt-cells':
+    const: 2
+
+  interrupts:
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+  - interrupt-controller
+  - '#interrupt-cells'
+
+additionalProperties: false
+
+examples:
+  - |
+    ioapic1: interrupt-controller@fec00000 {
+        compatible = "intel,ce4100-ioapic";
+        reg = <0xfec00000 0x1000>;
+        interrupt-controller;
+        #interrupt-cells = <2>;
+    };

Documentation/devicetree/bindings/interrupt-controller/intel,ce4100-lapic.yaml

@@ -0,0 +1,71 @@
+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: "http://devicetree.org/schemas/interrupt-controller/intel,ce4100-lapic.yaml#"
+$schema: "http://devicetree.org/meta-schemas/core.yaml#"
+
+title: Intel Local Advanced Programmable Interrupt Controller (LAPIC)
+
+maintainers:
+  - Rahul Tanwar <rtanwar@maxlinear.com>
+
+description: |
+  Intel's Advanced Programmable Interrupt Controller (APIC) is a
+  family of interrupt controllers. The APIC is a split
+  architecture design, with a local component (LAPIC) integrated
+  into the processor itself and an external I/O APIC. Local APIC
+  (lapic) receives interrupts from the processor's interrupt pins,
+  from internal sources and from an external I/O APIC (ioapic).
+  And it sends these to the processor core for handling.
+  See [1] Chapter 8 for more details.
+
+  Many of the Intel's generic devices like hpet, ioapic, lapic have
+  the ce4100 name in their compatible property names because they
+  first appeared in CE4100 SoC.
+
+  This schema defines bindings for local APIC interrupt controller.
+
+  [1] https://pdos.csail.mit.edu/6.828/2008/readings/ia32/IA32-3A.pdf
+
+properties:
+  compatible:
+    const: intel,ce4100-lapic
+
+  reg:
+    maxItems: 1
+
+  interrupt-controller: true
+
+  '#interrupt-cells':
+    const: 2
+
+  intel,virtual-wire-mode:
+    description: Intel defines a few possible interrupt delivery
+      modes. With respect to boot/init time, mainly two interrupt
+      delivery modes are possible.
+      PIC Mode - Legacy external 8259 compliant PIC interrupt controller.
+      Virtual Wire Mode - use lapic as virtual wire interrupt delivery mode.
+      For ACPI or MPS spec compliant systems, it is figured out by some read
+      only bit field/s available in their respective defined data structures.
+      For OF based systems, it is by default set to PIC mode.
+      But if this optional boolean property is set, then the interrupt delivery
+      mode is configured to virtual wire compatibility mode.
+    type: boolean
+
+required:
+  - compatible
+  - reg
+  - interrupt-controller
+  - '#interrupt-cells'
+
+additionalProperties: false
+
+examples:
+  - |
+    lapic0: interrupt-controller@fee00000 {
+        compatible = "intel,ce4100-lapic";
+        reg = <0xfee00000 0x1000>;
+        interrupt-controller;
+        #interrupt-cells = <2>;
+        intel,virtual-wire-mode;
+    };

arch/x86/Kconfig

@@ -463,8 +463,8 @@ config X86_X2APIC
 
 	  Some Intel systems circa 2022 and later are locked into x2APIC mode
 	  and can not fall back to the legacy APIC modes if SGX or TDX are
-	  enabled in the BIOS.  They will be unable to boot without enabling
-	  this option.
+	  enabled in the BIOS. They will boot with very reduced functionality
+	  without enabling this option.
 
 	  If you don't know what to do here, say N.
 

arch/x86/include/asm/apic.h

@@ -249,7 +249,6 @@ static inline u64 native_x2apic_icr_read(void)
 extern int x2apic_mode;
 extern int x2apic_phys;
 extern void __init x2apic_set_max_apicid(u32 apicid);
-extern void __init check_x2apic(void);
 extern void x2apic_setup(void);
 static inline int x2apic_enabled(void)
 {
@@ -258,13 +257,13 @@ static inline int x2apic_enabled(void)
 
 #define x2apic_supported()	(boot_cpu_has(X86_FEATURE_X2APIC))
 #else /* !CONFIG_X86_X2APIC */
-static inline void check_x2apic(void) { }
 static inline void x2apic_setup(void) { }
 static inline int x2apic_enabled(void) { return 0; }
 
 #define x2apic_mode		(0)
 #define	x2apic_supported()	(0)
 #endif /* !CONFIG_X86_X2APIC */
+extern void __init check_x2apic(void);
 
 struct irq_data;
 

arch/x86/kernel/apic/apic.c

@@ -1931,16 +1931,19 @@ void __init check_x2apic(void)
 	}
 }
 #else /* CONFIG_X86_X2APIC */
-static int __init validate_x2apic(void)
+void __init check_x2apic(void)
 {
 	if (!apic_is_x2apic_enabled())
-		return 0;
+		return;
 	/*
-	 * Checkme: Can we simply turn off x2apic here instead of panic?
+	 * Checkme: Can we simply turn off x2APIC here instead of disabling the APIC?
 	 */
-	panic("BIOS has enabled x2apic but kernel doesn't support x2apic, please disable x2apic in BIOS.\n");
+	pr_err("Kernel does not support x2APIC, please recompile with CONFIG_X86_X2APIC.\n");
+	pr_err("Disabling APIC, expect reduced performance and functionality.\n");
+
+	disable_apic = 1;
+	setup_clear_cpu_cap(X86_FEATURE_APIC);
 }
-early_initcall(validate_x2apic);
 
 static inline void try_to_enable_x2apic(int remap_mode) { }
 static inline void __x2apic_enable(void) { }

arch/x86/kernel/devicetree.c

@@ -31,11 +31,6 @@ char __initdata cmd_line[COMMAND_LINE_SIZE];
 
 int __initdata of_ioapic;
 
-void __init early_init_dt_add_memory_arch(u64 base, u64 size)
-{
-	BUG();
-}
-
 void __init add_dtb(u64 data)
 {
 	initial_dtb = data + offsetof(struct setup_data, data);
@@ -167,7 +162,14 @@ static void __init dtb_lapic_setup(void)
 			return;
 	}
 	smp_found_config = 1;
-	pic_mode = 1;
+	if (of_property_read_bool(dn, "intel,virtual-wire-mode")) {
+		pr_info("Virtual Wire compatibility mode.\n");
+		pic_mode = 0;
+	} else {
+		pr_info("IMCR and PIC compatibility mode.\n");
+		pic_mode = 1;
+	}
+
 	register_lapic_address(lapic_addr);
 }
 
@@ -248,7 +250,7 @@ static void __init dtb_add_ioapic(struct device_node *dn)
 
 	ret = of_address_to_resource(dn, 0, &r);
 	if (ret) {
-		printk(KERN_ERR "Can't obtain address from device node %pOF.\n", dn);
+		pr_err("Can't obtain address from device node %pOF.\n", dn);
 		return;
 	}
 	mp_register_ioapic(++ioapic_id, r.start, gsi_top, &cfg);
@@ -265,7 +267,7 @@ static void __init dtb_ioapic_setup(void)
 		of_ioapic = 1;
 		return;
 	}
-	printk(KERN_ERR "Error: No information about IO-APIC in OF.\n");
+	pr_err("Error: No information about IO-APIC in OF.\n");
 }
 #else
 static void __init dtb_ioapic_setup(void) {}