Notes about iommu=pt kernel parameter

当使用KVM pass-thru设备时，通常会设置intel_iommu=on iommu=pt内核参数，其中intel_iommu=on就是使能intel iommu，本文将介绍iommu=pt。

本文参考的内核版本是v5.0。

identity mapping指的是iova与hpa 1:1映射。

1. Motivation

The pt option only enables IOMMU translation for devices used in pass-thru ,doesn’t enable IOMMU translation for host used devices ,and this will improve performance for host PCIe devices (which are not pass-thru to a VM).

内核的注释：

/*
 * This variable becomes 1 if iommu=pt is passed on the kernel command line.
 * If this variable is 1, IOMMU implementations do no DMA translation for
 * devices and allow every device to access to whole physical memory. This is
 * useful if a user wants to use an IOMMU only for KVM device assignment to
 * guests and not for driver dma translation.
 */
int iommu_pass_through __read_mostly = 1;

2. 源码解析

2.1 `pt` option解析

static __init int iommu_setup(char *p)
{
    ...
    if (!strncmp(p, "pt", 2))
        iommu_pass_through = 1;
    ...
}

2.2 init_dmars

#define IDENTMAP_ALL        1


/*
 * This domain is a statically identity mapping domain.
 *  1. This domain creats a static 1:1 mapping to all usable memory.
 *  2. It maps to each iommu if successful.
 *  3. Each iommu maps to this domain if successful.
 */
static struct dmar_domain *si_domain;
static int hw_pass_through = 1;

static int __init init_dmars(void)
{
    ...
    if (!ecap_pass_through(iommu->ecap))
        hw_pass_through = 0;
    ...
    if (iommu_pass_through)
        iommu_identity_mapping |= IDENTMAP_ALL;
    ...
    if (iommu_identity_mapping) {
        ret = si_domain_init(hw_pass_through);
        if (ret)
            goto free_iommu;
    }
    ...
    if (iommu_identity_mapping) {
        ret = iommu_prepare_static_identity_mapping(hw_pass_through);
        if (ret) {
            pr_crit("Failed to setup IOMMU pass-through\n");
            goto free_iommu;
        }
    }
    ...
}

#define ecap_pass_through(e) ((e >> 6) & 0x1)

ecap_pass_through(iommu->ecap)的含义是检查Extended Capability Register的PT field。

如果Hardware supports pass-through translation type,那么hw_pass_through为1；否则hw_pass_through为0。

当iommu_pass_through被设置时，iommu_identity_mapping也会被设置。接着会依次调用si_domain_init与iommu_prepare_static_identity_mapping。

2.3 si_domain_init

static int __init si_domain_init(int hw)
{
    int nid, ret = 0;

    si_domain = alloc_domain(DOMAIN_FLAG_STATIC_IDENTITY);
    if (!si_domain)
        return -EFAULT;

    if (md_domain_init(si_domain, DEFAULT_DOMAIN_ADDRESS_WIDTH)) {
        domain_exit(si_domain);
        return -EFAULT;
    }

    pr_debug("Identity mapping domain allocated\n");

    if (hw)
        return 0;

    for_each_online_node(nid) { //迭代所有的活动结点(针对NUMA)
        unsigned long start_pfn, end_pfn;
        int i;

        for_each_mem_pfn_range(i, nid, &start_pfn, &end_pfn, NULL) {
            ret = iommu_domain_identity_map(si_domain, //iova与hpa 1:1映射
                    PFN_PHYS(start_pfn), PFN_PHYS(end_pfn));
            if (ret)
                return ret;
        }
    }

    return 0;
}

从上述代码可知，当hw_pass_through为1时，无需建立iova与hpa 1:1映射的iommu页表；否则需要对all usable memory建立iova与hpa 1:1映射的iommu页表。

2.4 iommu_prepare_static_identity_mapping

iommu_prepare_static_identity_mapping
└── dev_prepare_static_identity_mapping
    └── domain_add_dev_info
        └── dmar_insert_one_dev_info
            └── domain_context_mapping
                └── domain_context_mapping_one

static int domain_context_mapping_one(struct dmar_domain *domain,
                                      struct intel_iommu *iommu,
                                      u8 bus, u8 devfn)
{
    ...
    // 设置translation type 为 pass through
    if (hw_pass_through && domain_type_is_si(domain))
        translation = CONTEXT_TT_PASS_THROUGH;
    ...
    // 获取这个设备在contex table表里面的地址
    context = iommu_context_addr(iommu, bus, devfn, 1);
    ...
    struct dma_pte *pgd = domain->pgd; // iova页表基址
    int agaw;

    context_set_domain_id(context, did);
    // 设置转换类型
    context_set_translation_type(context, translation);

    // 下面代码可以看出pass through模式不会设置iova页表地址
    if (translation != CONTEXT_TT_PASS_THROUGH) {
        ...
        // 非pass through模式下需要设置iova页表的基地址
        context_set_address_root(context, virt_to_phys(pgd));
        context_set_address_width(context, agaw);
    } else {
        /*
         * In pass through mode, AW must be programmed to
         * indicate the largest AGAW value supported by
         * hardware. And ASR is ignored by hardware.
         */
        context_set_address_width(context, iommu->msagaw);
    }
    ...
}

#define CONTEXT_TT_PASS_THROUGH 2

因此CONTEXT_TT_PASS_THROUGH为10b，即是2。

3. 总结

配置了iommu=pt就会实现identity mapping:

如果Hardware supports pass-through translation type，则配置pass-through translation type即可实现identity mapping，此时无需配置iommu页表;
如果Hardware doesn’t support pass-through translation type，则需要配置iommu页表，使得iova与hpa 1:1映射。

当hw_pass_through=0时，依然要走iommu页表，因此性能是不如hw_pass_through=1的。

参考资料:

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