x86/fred: Install system vector handlers even if FRED isn't fully enabled

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>
Link: https://lkml.kernel.org/r/20250714103441.121251108@infradead.org

diff --git a/arch/x86/include/asm/idtentry.h b/arch/x86/include/asm/idtentry.h
index a4ec27c6798875900cbdbba927918e70b900f63b..abd637e54e94e3d81cfde9435535d006457f6090 100644 (file)
--- a/arch/x86/include/asm/idtentry.h
+++ b/arch/x86/include/asm/idtentry.h
@@ -460,17 +460,12 @@ __visible noinstr void func(struct pt_regs *regs,                 \
 #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);             \
 }
 

diff --git a/arch/x86/kernel/irqinit.c b/arch/x86/kernel/irqinit.c
index f79c5edc0b892da8e3dff9afda2324ae0dae5944..6ab9eac64670acf8ce8a96dc78c4649057b8907e 100644 (file)
--- 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();