[PATCH v3 13/16] x86/fred: Install system vector handlers even if FRED isnt fully enabled

[Peter Zijlstra <peterz@infradead.org>]

From: Sean Christopherson <seanjc@google.com>

Install the system vector IRQ handlers for FRED even if FRED isn't fully
enabled in hardware.  This will allow KVM to use the FRED IRQ path even
on non-FRED hardware, which in turn will eliminate a non-CFI indirect CALL
(KVM currently invokes the IRQ handler via an IDT lookup on the vector).

[sean: extract from diff, drop stub, write changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/include/asm/idtentry.h |    9 ++-------
 arch/x86/kernel/irqinit.c       |    6 ++++--
 2 files changed, 6 insertions(+), 9 deletions(-)

--- a/arch/x86/include/asm/idtentry.h
+++ b/arch/x86/include/asm/idtentry.h
@@ -460,17 +460,12 @@ __visible noinstr void func(struct pt_re
 #endif
 
 void idt_install_sysvec(unsigned int n, const void *function);
-
-#ifdef CONFIG_X86_FRED
 void fred_install_sysvec(unsigned int vector, const idtentry_t function);
-#else
-static inline void fred_install_sysvec(unsigned int vector, const idtentry_t function) { }
-#endif
 
 #define sysvec_install(vector, function) {				\
-	if (cpu_feature_enabled(X86_FEATURE_FRED))			\
+	if (IS_ENABLED(CONFIG_X86_FRED))				\
 		fred_install_sysvec(vector, function);			\
-	else								\
+	if (!cpu_feature_enabled(X86_FEATURE_FRED))			\
 		idt_install_sysvec(vector, asm_##function);		\
 }
 
--- a/arch/x86/kernel/irqinit.c
+++ b/arch/x86/kernel/irqinit.c
@@ -97,9 +97,11 @@ void __init native_init_IRQ(void)
 	/* Execute any quirks before the call gates are initialised: */
 	x86_init.irqs.pre_vector_init();
 
-	if (cpu_feature_enabled(X86_FEATURE_FRED))
+	/* FRED's IRQ path may be used even if FRED isn't fully enabled. */
+	if (IS_ENABLED(CONFIG_X86_FRED))
 		fred_complete_exception_setup();
-	else
+
+	if (!cpu_feature_enabled(X86_FEATURE_FRED))
 		idt_setup_apic_and_irq_gates();
 
 	lapic_assign_system_vectors();