irq_remap_ops定义在drivers\iommu\irq_remapping.h中：
struct irq_remap_ops {
/* Check whether Interrupt Remapping is supported */
int (*supported)(void);
/* Initializes hardware and makes it ready for remapping interrupts */
int (*prepare)(void);
/* Enables the remapping hardware */
int (*enable)(void);
/* Disables the remapping hardware */
void (*disable)(void);
/* Reenables the remapping hardware */
int (*reenable)(int);
/* Enable fault handling */
int (*enable_faulting)(void);
/* IO-APIC setup routine */
int (*setup_ioapic_entry)(int irq, struct IO_APIC_route_entry *,
unsigned int, int,
struct io_apic_irq_attr *);
/* Set the CPU affinity of a remapped interrupt */
int (*set_affinity)(struct irq_data *data, const struct cpumask *mask,
bool force);
/* Free an IRQ */
int (*free_irq)(int);
/* Create MSI msg to use for interrupt remapping */
void (*compose_msi_msg)(struct pci_dev *,
unsigned int, unsigned int,
struct msi_msg *, u8);
/* Allocate remapping resources for MSI */
int (*msi_alloc_irq)(struct pci_dev *, int, int);
/* Setup the remapped MSI irq */
int (*msi_setup_irq)(struct pci_dev *, unsigned int, int, int);
/* Setup interrupt remapping for an HPET MSI */
int (*setup_hpet_msi)(unsigned int, unsigned int);
};
extern struct irq_remap_ops intel_irq_remap_ops;
extern struct irq_remap_ops amd_iommu_irq_ops;
可以看到，在结构体中定义了一系列的函数指针，每个函数的作用都写得很清楚。
针对intel处理器的结构体定义在drivers\iommu\intel_irq_remapping.c：
struct irq_remap_ops intel_irq_remap_ops = {
.supported = intel_irq_remapping_supported,
.prepare = dmar_table_init,
.enable = intel_enable_irq_remapping,
.disable = disable_irq_remapping,
.reenable = reenable_irq_remapping,
.enable_faulting = enable_drhd_fault_handling,
.setup_ioapic_entry = intel_setup_ioapic_entry,
.set_affinity = intel_ioapic_set_affinity,
.free_irq = free_irte,
.compose_msi_msg = intel_compose_msi_msg,
.msi_alloc_irq = intel_msi_alloc_irq,
.msi_setup_irq = intel_msi_setup_irq,
.setup_hpet_msi = intel_setup_hpet_msi,
};
AMD相关的暂不讨论。
在enable_IR_x2apic函数（arch\x86\kernel\apic）中，包含着irq_remap_ops中初始化相关函数的调用：
void __init enable_IR_x2apic(void)
{
......
/* Make sure irq_remap_ops are initialized */
setup_irq_remapping_ops();
hardware_init_ret = irq_remapping_prepare();
if (hardware_init_ret && !x2apic_supported())
return;
......
if (hardware_init_ret)
ret = -1;
else
ret = enable_IR();
......
}
（1）
void __init setup_irq_remapping_ops(void)
{
remap_ops = &intel_irq_remap_ops;
#ifdef CONFIG_AMD_IOMMU
if (amd_iommu_irq_ops.prepare() == 0)
remap_ops = &amd_iommu_irq_ops;
#endif
}
这个函数会让全局变量remap_ops这个指针指向intel_irq_remap_ops或amd_iommu_irq_ops。
（2）
int __init irq_remapping_prepare(void)
{
if (!remap_ops || !remap_ops->prepare)
return -ENODEV;
return remap_ops->prepare();
}
irq_remapping_prepare会调用prepare函数（Intel平台即dmar_table_init）。
（3）
int __init enable_IR(void)
{
#ifdef CONFIG_IRQ_REMAP
if (!irq_remapping_supported()) {
pr_debug("intr-remapping not supported\n");
return -1;
}
if (!x2apic_preenabled && skip_ioapic_setup) {
pr_info("Skipped enabling intr-remap because of skipping "
"io-apic setup\n");
return -1;
}
return irq_remapping_enable();
#endif
return -1;
}
在enable_IR中会调用irq_remapping_supported和irq_remapping_enable：
int irq_remapping_supported(void)
{
if (disable_irq_remap)
return 0;
if (!remap_ops || !remap_ops->supported)
return 0;
return remap_ops->supported();
}
int __init irq_remapping_enable(void)
{
int ret;
if (!remap_ops || !remap_ops->enable)
return -ENODEV;
ret = remap_ops->enable();
if (irq_remapping_enabled)
irq_remapping_modify_x86_ops();
return ret;
}
会先后调用supported（Intel平台即为intel_irq_remapping_supported）和enable函数（Intel平台即为intel_enable_irq_remapping）。
irq_remapping_modify_x86_ops会把其它函数赋值给相应的全局函数指针：
static void __init irq_remapping_modify_x86_ops(void)
{
x86_io_apic_ops.disable = irq_remapping_disable_io_apic;
x86_io_apic_ops.set_affinity = set_remapped_irq_affinity;
x86_io_apic_ops.setup_entry = setup_ioapic_remapped_entry;
x86_io_apic_ops.eoi_ioapic_pin = eoi_ioapic_pin_remapped;
x86_msi.setup_msi_irqs = irq_remapping_setup_msi_irqs;
x86_msi.setup_hpet_msi = setup_hpet_msi_remapped;
x86_msi.compose_msi_msg = compose_remapped_msi_msg;
}