Xen devel - May 2009 - what is the aim of "pinning" a pagetable?

hi,gurus
I am getting confused by the meaning of "pinning a pagetable" in a
para-virtualized guest.
according to the comments in source code

""""
Naively, when loading %cr3 with the base of a new pagetable, Xen
 * would need to validate the whole pagetable before going on.
 * Naturally, this is quite slow.  The solution is to "pin" a
 * pagetable, which enforces all the constraints on the pagetable even
 * when it is not actively in use.  This menas that Xen can be assured
 * that it is still valid when you do load it into %cr3, and doesn''t
 * need to revalidate it.
""""

I have two questions on these .

1. does pinning a pagetable means make this page readonly so no changes would
be made in it(even when it is not actively in use).
After that when we load it into %cr3 there is no need to revalidate it.

2. what kind of action would the hypervisor take while validating the
whole pagetable?
my understanding is to check if the pagetables are all marked as
readonly and make sure that all machine addreses contained in
pagetables are valid.
am I correct?

any explanation is appreciated
thanks

lei

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