Writable page tables questions

[Junji Zhi <junji.xendev@gmail.com>]

Hi,

I'm Junji, a newbie in Xen and hoping I can contribute to the community 
one day. I have a few questions regarding the writable page tables, 
while reading The Definitive Guide to the Xen Hypervisor by David Chisnall:

1. Writable page tables is one Xen memory assist technique, applied to 
paravirtualized guests ONLY. HVM does not apply. Correct?

2. According to the book, when a guest wants to modify its page table, 
it triggers a trap into the hypervisor and it does a few steps:

(1) it invalidates a PTE that points to the page containing the page 
table. Is my understanding correct?

Q: What does "invalidate" really mean here? Does it mean simply flipping 
a bit in the PTE of the page table, or removing the PTE completely? Does 
it also need to invalidate the TLB entry?

(2) then the control goes back to the guest and it can write/read the 
page table now.

(3) The book's words pasted: "When an address referenced by the newly 
invalidated page directory entry is referenced (read or write), a page 
fault occurs. "

Q: The description of step (3) is confusing. What does it mean by "an 
address referenced by the newly invalidated page directory entry is 
referenced"? Does it mean the case when the guest code is accessing an 
virtual address that needs to search the invalidated page table for 
translation?


Thanks and I really appreciate any comment or responses.
Junji