x86/irq: Reserve a per CPU IDT vector for posted MSIs

When posted MSI is enabled, all device MSIs are multiplexed into a single
notification vector. MSI handlers will be de-multiplexed at run-time by
system software without IDT delivery.
Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240423174114.526704-6-jacob.jun.pan@linux.intel.com

arch/x86/include/asm/irq_vectors.h

@@ -97,10 +97,16 @@
 
 #define LOCAL_TIMER_VECTOR		0xec
 
+/*
+ * Posted interrupt notification vector for all device MSIs delivered to
+ * the host kernel.
+ */
+#define POSTED_MSI_NOTIFICATION_VECTOR	0xeb
+
 #define NR_VECTORS			 256
 
 #ifdef CONFIG_X86_LOCAL_APIC
-#define FIRST_SYSTEM_VECTOR		LOCAL_TIMER_VECTOR
+#define FIRST_SYSTEM_VECTOR		POSTED_MSI_NOTIFICATION_VECTOR
 #else
 #define FIRST_SYSTEM_VECTOR		NR_VECTORS
 #endif