[Xen-devel] one question on __user_pgd

[leilei175@xxxxxxxxx]

in the mm_pin routine,
            the function would do

        mm_walk(mm, PAGE_KERNEL_RO);
        xen_pgd_pin(__pa(mm->pgd)); /* kernel */
        xen_pgd_pin(__pa(__user_pgd(mm->pgd))); /* user */
        mm->context.pinned = 1;

then I saw the macro __user_pgd is defined as

#define __user_pgd(pgd) ((pgd) + PTRS_PER_PGD)

my question is:
what's the use of __user_pgd?
moreover, concerning the action of "pin" the page directory table,
what is the use of it?

could some one shed some light on these questions? Any explanation is
appreciated.

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