iommu/vt-d: Make posted MSI an opt-in command line option

Add a command line opt-in option for posted MSI if CONFIG_X86_POSTED_MSI=y.

Also introduce a helper function for testing if posted MSI is supported on
the platform.
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/20240423174114.526704-12-jacob.jun.pan@linux.intel.com

Documentation/admin-guide/kernel-parameters.txt

@@ -2251,6 +2251,8 @@
 			no_x2apic_optout
 				BIOS x2APIC opt-out request will be ignored
 			nopost	disable Interrupt Posting
+			posted_msi
+				enable MSIs delivered as posted interrupts
 
 	iomem=		Disable strict checking of access to MMIO memory
 		strict	regions from userspace.

arch/x86/include/asm/irq_remapping.h

@@ -50,6 +50,13 @@ static inline struct irq_domain *arch_get_ir_parent_domain(void)
 	return x86_vector_domain;
 }
 
+extern bool enable_posted_msi;
+
+static inline bool posted_msi_supported(void)
+{
+	return enable_posted_msi && irq_remapping_cap(IRQ_POSTING_CAP);
+}
+
 #else  /* CONFIG_IRQ_REMAP */
 
 static inline bool irq_remapping_cap(enum irq_remap_cap cap) { return 0; }

drivers/iommu/irq_remapping.c

@@ -24,6 +24,8 @@ int no_x2apic_optout;
 
 int disable_irq_post = 0;
 
+bool enable_posted_msi __ro_after_init;
+
 static int disable_irq_remap;
 static struct irq_remap_ops *remap_ops;
 
@@ -70,7 +72,8 @@ static __init int setup_irqremap(char *str)
 			no_x2apic_optout = 1;
 		else if (!strncmp(str, "nopost", 6))
 			disable_irq_post = 1;
-
+		else if (IS_ENABLED(CONFIG_X86_POSTED_MSI) && !strncmp(str, "posted_msi", 10))
+			enable_posted_msi = true;
 		str += strcspn(str, ",");
 		while (*str == ',')
 			str++;