x86/fred: Move FRED RSP initialization into a separate function

To enable FRED earlier, move the RSP initialization out of
cpu_init_fred_exceptions() into cpu_init_fred_rsps().

This is required as the FRED RSP initialization depends on the availability
of the CPU entry areas which are set up late in trap_init(),

No functional change intended. Marked with Fixes as it's a depedency for
the real fix.

Fixes: 14619d9 ("x86/fred: FRED entry/exit and dispatch code")
Signed-off-by: Xin Li (Intel) <xin@zytor.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20240709154048.3543361-3-xin@zytor.com

Author: xinli-intel

arch/x86/include/asm/fred.h

@@ -84,11 +84,13 @@ static __always_inline void fred_entry_from_kvm(unsigned int type, unsigned int
 }
 
 void cpu_init_fred_exceptions(void);
+void cpu_init_fred_rsps(void);
 void fred_complete_exception_setup(void);
 
 #else /* CONFIG_X86_FRED */
 static __always_inline unsigned long fred_event_data(struct pt_regs *regs) { return 0; }
 static inline void cpu_init_fred_exceptions(void) { }
+static inline void cpu_init_fred_rsps(void) { }
 static inline void fred_complete_exception_setup(void) { }
 static __always_inline void fred_entry_from_kvm(unsigned int type, unsigned int vector) { }
 #endif /* CONFIG_X86_FRED */

arch/x86/kernel/cpu/common.c

@@ -2195,10 +2195,12 @@ void cpu_init_exception_handling(void)
 	/* GHCB needs to be setup to handle #VC. */
 	setup_ghcb();
 
-	if (cpu_feature_enabled(X86_FEATURE_FRED))
+	if (cpu_feature_enabled(X86_FEATURE_FRED)) {
 		cpu_init_fred_exceptions();
-	else
+		cpu_init_fred_rsps();
+	} else {
 		load_current_idt();
+	}
 }
 
 /*

arch/x86/kernel/fred.c

@@ -32,6 +32,25 @@ void cpu_init_fred_exceptions(void)
 	       FRED_CONFIG_INT_STKLVL(0) |
 	       FRED_CONFIG_ENTRYPOINT(asm_fred_entrypoint_user));
 
+	wrmsrl(MSR_IA32_FRED_STKLVLS, 0);
+	wrmsrl(MSR_IA32_FRED_RSP0, 0);
+	wrmsrl(MSR_IA32_FRED_RSP1, 0);
+	wrmsrl(MSR_IA32_FRED_RSP2, 0);
+	wrmsrl(MSR_IA32_FRED_RSP3, 0);
+
+	/* Enable FRED */
+	cr4_set_bits(X86_CR4_FRED);
+	/* Any further IDT use is a bug */
+	idt_invalidate();
+
+	/* Use int $0x80 for 32-bit system calls in FRED mode */
+	setup_clear_cpu_cap(X86_FEATURE_SYSENTER32);
+	setup_clear_cpu_cap(X86_FEATURE_SYSCALL32);
+}
+
+/* Must be called after setup_cpu_entry_areas() */
+void cpu_init_fred_rsps(void)
+{
 	/*
 	 * The purpose of separate stacks for NMI, #DB and #MC *in the kernel*
 	 * (remember that user space faults are always taken on stack level 0)
@@ -47,13 +66,4 @@ void cpu_init_fred_exceptions(void)
 	wrmsrl(MSR_IA32_FRED_RSP1, __this_cpu_ist_top_va(DB));
 	wrmsrl(MSR_IA32_FRED_RSP2, __this_cpu_ist_top_va(NMI));
 	wrmsrl(MSR_IA32_FRED_RSP3, __this_cpu_ist_top_va(DF));
-
-	/* Enable FRED */
-	cr4_set_bits(X86_CR4_FRED);
-	/* Any further IDT use is a bug */
-	idt_invalidate();
-
-	/* Use int $0x80 for 32-bit system calls in FRED mode */
-	setup_clear_cpu_cap(X86_FEATURE_SYSENTER32);
-	setup_clear_cpu_cap(X86_FEATURE_SYSCALL32);
 }