看一下intel_irq_remapping_supported的代码:

static int __init intel_irq_remapping_supported(void)
{
    struct dmar_drhd_unit *drhd;
    struct intel_iommu *iommu;

    if (disable_irq_remap)
        return 0;
    if (irq_remap_broken) {
        printk(KERN_WARNING
            "This system BIOS has enabled interrupt remapping\n"
            "on a chipset that contains an erratum making that\n"
            "feature unstable.  To maintain system stability\n"
            "interrupt remapping is being disabled.  Please\n"
            "contact your BIOS vendor for an update\n");
        add_taint(TAINT_FIRMWARE_WORKAROUND, LOCKDEP_STILL_OK);
        disable_irq_remap = 1;
        return 0;
    }

    if (!dmar_ir_support())
        return 0;

    for_each_iommu(iommu, drhd)
        if (!ecap_ir_support(iommu->ecap))
            return 0;

    return 1;
}

disable_irq_remapirq_remap_broken定义在irq_remapping.c中,用来判断是否支持IRQ remapping这个功能。

dmar_ir_support定义在dmar.c中:

/*
 * Check interrupt remapping support in DMAR table description.
 */
int __init dmar_ir_support(void)
{
    struct acpi_table_dmar *dmar;
    dmar = (struct acpi_table_dmar *)dmar_tbl;
    if (!dmar)
        return 0;
    return dmar->flags & 0x1;
}

flags的第0位标示平台是否支持IRQ remapping这个功能。

    for_each_iommu(iommu, drhd)
        if (!ecap_ir_support(iommu->ecap))
            return 0;

这段代码检查DMA Remapping Hardware Unit的所代表的IOMMU单元是否支持IRQ remapping这个功能。

如果检查都通过了,返回1,表示支持IRQ remapping这个功能。

参考资料:
"BIOS Considerations" in *Intel ® Virtualization Technology for Directed I/O