Re: How does shadow page table work during migration?

[Kevin Negy <kevinnegy@xxxxxxxxx>]

Hello again,

Thank you for the helpful responses. I have several follow up questions.

1)

With Shadow, Xen has to do the combination of address spaces itself -
the shadow pagetables map guest virtual to host physical address.

The shadow_blow_tables() call is "please recycle everything" which is used
to throw away all shadow pagetables, which in turn will cause the
shadows to be recreated from scratch as the guest continues to run. 

With shadowing enabled, given a guest virtual address, how does the hypervisor recreate the mapping to the host physical address (mfn) from the virtual address if the shadow page tables are empty (after a call to shadow_blow_tables, for instance)? I had been thinking of shadow page tables as the definitive mapping between guest pages and machine pages, but should I think of them as more of a TLB, which implies there's another way to get/recreate the mappings if there's no entry in the shadow table?

2) I'm trying to grasp the general steps of enabling shadowing and handling page faults. Is this correct?
    a) Normal PV - default shadowing is disabled, guest has its page tables in r/w mode or whatever mode is considered normal for guest page tables
    b) Shadowing is enabled - shadow memory pool allocated, all memory accesses must now go through shadow pages in CR3. Since no entries are in shadow tables, initial read and writes from the guest will result in page faults.
    c) As soon as the first guest memory access occurs, a mandatory page fault occurs because there is no mapping in the shadows. Xen does a guest page table walk for the address that caused the fault (va) and then marks all the guest page table pages along the walk as read only.
    d) Xen finds out the mfn of the guest va somehow (my first question) and adds the mapping of the va to the shadow page table.
    e) If the page fault was a write, the va is now marked as read/write but logged as dirty in the logdirty map.
    e) Now the next page fault to any of the page tables marked read-only in c) must have been caused by the guest writing to its tables, which can be reflected in the shadow page tables.
   

3) How do Xen/shadow page tables distinguish between two equivalent guest virtual addresses from different guest processes? I suppose when a guest OS tries to change page tables from one process to another, this will cause a page fault that Xen will trap and be able to infer that the current shadow page table should be swapped to a different one corresponding to the new guest process?

Thank you so much,

Kevin