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

["tip-bot2 for Sean Christopherson" <tip-bot2@linutronix.de>]

The following commit has been merged into the x86/core branch of tip:

Commit-ID:     2d1435b742156f6d4adc8b9ab0f24b35d879266e
Gitweb:        https://git.kernel.org/tip/2d1435b742156f6d4adc8b9ab0f24b35d879266e
Author:        Sean Christopherson <seanjc@google.com>
AuthorDate:    Fri, 02 May 2025 07:24:01 -07:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Mon, 18 Aug 2025 14:23:08 +02:00

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
---
 arch/x86/include/asm/idtentry.h |  9 ++-------
 arch/x86/kernel/irqinit.c       |  6 ++++--
 2 files changed, 6 insertions(+), 9 deletions(-)

diff --git a/arch/x86/include/asm/idtentry.h b/arch/x86/include/asm/idtentry.h
index a4ec27c..abd637e 100644
--- 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 f79c5ed..6ab9eac 100644
--- 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();