Re: [PATCH] x86/PV: make post-migration page state consistent

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 11/09/2020 11:34, Jan Beulich wrote:
> When a page table page gets de-validated, its type reference count drops
> to zero (and PGT_validated gets cleared), but its type remains intact.
> XEN_DOMCTL_getpageframeinfo3, therefore, so far reported prior usage for
> such pages. An intermediate write to such a page via e.g.
> MMU_NORMAL_PT_UPDATE, however, would transition the page's type to
> PGT_writable_page, thus altering what XEN_DOMCTL_getpageframeinfo3 would
> return. In libxc the decision which pages to normalize / localize
> depends solely on the type returned from the domctl. As a result without
> further precautions the guest won't be able to tell whether such a page
> has had its (apparent) PTE entries transitioned to the new MFNs.

I'm afraid I don't follow what the problem is.

Yes - unvalidated pages probably ought to be consistently NOTAB, so this
is probably a good change, but I don't see how it impacts the migration
logic.

We already have to cope with a page really changing types in parallel
with the normalise/localise logic (that was a "fun" one to debug), which
is why errors in that logic are specifically not fatal while the guest
is live - the frame gets re-marked as dirty, and deferred until the next
round.

Errors encountered after the VM has been paused are fatal.

However, at no point, even with an unvalidated pagetable type, can the
contents of the page be anything other than legal PTEs.  (I think)

~Andrew