root_entry3.10版本的相关定义:

/*
 * 0: Present
 * 1-11: Reserved
 * 12-63: Context Ptr (12 - (haw-1))
 * 64-127: Reserved
 */
struct root_entry {
    u64 val;
    u64 rsvd1;
};
#define ROOT_ENTRY_NR (VTD_PAGE_SIZE/sizeof(struct root_entry))
static inline bool root_present(struct root_entry *root)
{
    return (root->val & 1);
}
static inline void set_root_present(struct root_entry *root)
{
    root->val |= 1;
}
static inline void set_root_value(struct root_entry *root, unsigned long value)
{
    root->val |= value & VTD_PAGE_MASK;
}

root_entrymainstream版本的相关定义:

/*
 * 0: Present
 * 1-11: Reserved
 * 12-63: Context Ptr (12 - (haw-1))
 * 64-127: Reserved
 */
struct root_entry {
    u64 lo;
    u64 hi;
};
#define ROOT_ENTRY_NR (VTD_PAGE_SIZE/sizeof(struct root_entry))

/*
 * Take a root_entry and return the Lower Context Table Pointer (LCTP)
 * if marked present.
 */
static phys_addr_t root_entry_lctp(struct root_entry *re)
{
    if (!(re->lo & 1))
        return 0;

    return re->lo & VTD_PAGE_MASK;
}

/*
 * Take a root_entry and return the Upper Context Table Pointer (UCTP)
 * if marked present.
 */
static phys_addr_t root_entry_uctp(struct root_entry *re)
{
    if (!(re->hi & 1))
        return 0;

    return re->hi & VTD_PAGE_MASK;
}

VTD_PAGE_MASK的相关定义:

/*
 * VT-d hardware uses 4KiB page size regardless of host page size.
 */
#define VTD_PAGE_SHIFT      (12)
#define VTD_PAGE_SIZE       (1UL << VTD_PAGE_SHIFT)
#define VTD_PAGE_MASK       (((u64)-1) << VTD_PAGE_SHIFT)
#define VTD_PAGE_ALIGN(addr)    (((addr) + VTD_PAGE_SIZE - 1) & VTD_PAGE_MASK)

所以root_entry_lctp得到的是Context Table的物理地址。

Root entry的格式如下:

1

Extended root entry的格式如下:

2

Root entryextended root entry都占16byte16 * 8 = 128),而HAW代表这个平台的Host Address Width,一共有256root entryextended root entry4096/16 = 256)。

参考资料:
Intel ® Virtualization Technology for Directed I/O