Re: [PATCH v1 2/2] virtio-balloon: disallow postcopy with VIRTIO_BALLOON_F_FREE_PAGE_HINT

[Peter Xu <peterx@redhat.com>]

On Wed, Jul 07, 2021 at 08:57:29PM +0200, David Hildenbrand wrote:
> On 07.07.21 20:02, Peter Xu wrote:
> > On Wed, Jul 07, 2021 at 04:06:55PM +0200, David Hildenbrand wrote:
> > > As it never worked properly, let's disable it via the postcopy notifier on
> > > the destination. Trying to set "migrate_set_capability postcopy-ram on"
> > > on the destination now results in "virtio-balloon: 'free-page-hint' does
> > > not support postcopy Error: Postcopy is not supported".
> > 
> > Would it be possible to do this in reversed order?  Say, dynamically disable
> > free-page-hinting if postcopy capability is set when migration starts? Perhaps
> > it can also be re-enabled automatically when migration completes?
> 
> I remember that this might be quite racy. We would have to make sure that no
> hinting happens before we enable the capability.
> 
> As soon as we messed with the dirty bitmap (during precopy), postcopy is no
> longer safe. As noted in the patch, the only runtime alternative is to
> disable postcopy as soon as we actually do clear a bit. Alternatively, we
> could ignore any hints if the postcopy capability was enabled.

Logically migration capabilities are applied at VM starts, and these
capabilities should be constant during migration (I didn't check if there's a
hard requirement; easy to add that if we want to assure it), and in most cases
for the lifecycle of the vm.

> 
> Whatever we do, we have to make sure that a user cannot trick the system
> into an inconsistent state. Like enabling hinting, starting migration, then
> enabling the postcopy capability and kicking of postcopy. I did not check if
> we allow for that, though.

We could turn free page hinting off when migration starts with postcopy-ram=on,
then re-enable it after migration finishes.  That looks very safe to me.  And I
don't even worry on user trying to mess it up - as that only put their own VM
at risk; that's mostly fine to me.

Thanks,

-- 
Peter Xu