Re: [Qemu-devel] [PATCH v7 42/42] Inhibit ballooning during postcopy

[Juan Quintela]

"Dr. David Alan Gilbert (git)" <address@hidden> wrote:
> From: "Dr. David Alan Gilbert" <address@hidden>
>
> Postcopy detects accesses to pages that haven't been transferred yet
> using userfaultfd, and it causes exceptions on pages that are 'not
> present'.
> Ballooning also causes pages to be marked as 'not present' when the
> guest inflates the balloon.
> Potentially a balloon could be inflated to discard pages that are
> currently inflight during postcopy and that may be arriving at about
> the same time.
>
> To avoid this confusion, disable ballooning during postcopy.
>
> When disabled we drop balloon requests from the guest.  Since ballooning
> is generally initiated by the host, the management system should avoid
> initiating any balloon instructions to the guest during migration,
> although it's not possible to know how long it would take a guest to
> process a request made prior to the start of migration.
>
> Queueing the requests until after migration would be nice, but is
> non-trivial, since the set of inflate/deflate requests have to
> be compared with the state of the page to know what the final
> outcome is allowed to be.
>
> Signed-off-by: Dr. David Alan Gilbert <address@hidden>

Reviewed-by: Juan Quintela <address@hidden>