x86/fred: Invoke FRED initialization code to enable FRED

Let cpu_init_exception_handling() call cpu_init_fred_exceptions() to
initialize FRED. However if FRED is unavailable or disabled, it falls
back to set up TSS IST and initialize IDT.

Co-developed-by: Xin Li <xin3.li@intel.com>
Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Signed-off-by: Xin Li <xin3.li@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Tested-by: Shan Kang <shan.kang@intel.com>
Link: https://lore.kernel.org/r/20231205105030.8698-36-xin3.li@intel.com

Author: H. Peter Anvin (Intel)

arch/x86/kernel/cpu/common.c

@@ -61,6 +61,7 @@
 #include <asm/microcode.h>
 #include <asm/intel-family.h>
 #include <asm/cpu_device_id.h>
+#include <asm/fred.h>
 #include <asm/uv/uv.h>
 #include <asm/ia32.h>
 #include <asm/set_memory.h>
@@ -2107,7 +2108,15 @@ void syscall_init(void)
 	/* The default user and kernel segments */
 	wrmsr(MSR_STAR, 0, (__USER32_CS << 16) | __KERNEL_CS);
 
-	idt_syscall_init();
+	/*
+	 * Except the IA32_STAR MSR, there is NO need to setup SYSCALL and
+	 * SYSENTER MSRs for FRED, because FRED uses the ring 3 FRED
+	 * entrypoint for SYSCALL and SYSENTER, and ERETU is the only legit
+	 * instruction to return to ring 3 (both sysexit and sysret cause
+	 * #UD when FRED is enabled).
+	 */
+	if (!cpu_feature_enabled(X86_FEATURE_FRED))
+		idt_syscall_init();
 }
 
 #else	/* CONFIG_X86_64 */
@@ -2213,8 +2222,9 @@ void cpu_init_exception_handling(void)
 	/* paranoid_entry() gets the CPU number from the GDT */
 	setup_getcpu(cpu);
 
-	/* IST vectors need TSS to be set up. */
-	tss_setup_ist(tss);
+	/* For IDT mode, IST vectors need to be set in TSS. */
+	if (!cpu_feature_enabled(X86_FEATURE_FRED))
+		tss_setup_ist(tss);
 	tss_setup_io_bitmap(tss);
 	set_tss_desc(cpu, &get_cpu_entry_area(cpu)->tss.x86_tss);
 
@@ -2223,8 +2233,10 @@ void cpu_init_exception_handling(void)
 	/* GHCB needs to be setup to handle #VC. */
 	setup_ghcb();
 
-	/* Finally load the IDT */
-	load_current_idt();
+	if (cpu_feature_enabled(X86_FEATURE_FRED))
+		cpu_init_fred_exceptions();
+	else
+		load_current_idt();
 }
 
 /*

arch/x86/kernel/irqinit.c

@@ -28,6 +28,7 @@
 #include <asm/setup.h>
 #include <asm/i8259.h>
 #include <asm/traps.h>
+#include <asm/fred.h>
 #include <asm/prom.h>
 
 /*
@@ -96,7 +97,11 @@ void __init native_init_IRQ(void)
 	/* Execute any quirks before the call gates are initialised: */
 	x86_init.irqs.pre_vector_init();
 
-	idt_setup_apic_and_irq_gates();
+	if (cpu_feature_enabled(X86_FEATURE_FRED))
+		fred_complete_exception_setup();
+	else
+		idt_setup_apic_and_irq_gates();
+
 	lapic_assign_system_vectors();
 
 	if (!acpi_ioapic && !of_ioapic && nr_legacy_irqs()) {

arch/x86/kernel/traps.c

@@ -1438,7 +1438,10 @@ void __init trap_init(void)
 
 	/* Initialize TSS before setting up traps so ISTs work */
 	cpu_init_exception_handling();
+
 	/* Setup traps as cpu_init() might #GP */
-	idt_setup_traps();
+	if (!cpu_feature_enabled(X86_FEATURE_FRED))
+		idt_setup_traps();
+
 	cpu_init();
 }