Re: [PATCH v2] KVM: kvm_io_bus_unregister_dev() should never fail

[Cornelia Huck]

On Fri, 24 Mar 2017 09:55:15 +0100
David Hildenbrand <david@xxxxxxxxxx> wrote:

> 
> >>> -             return r;
> >>> +     if (i == bus->dev_count)
> >>> +             return;
> >>>
> >>>       new_bus = kmalloc(sizeof(*bus) + ((bus->dev_count - 1) *
> >>>                         sizeof(struct kvm_io_range)), GFP_KERNEL);
> >>> -     if (!new_bus)
> >>> -             return -ENOMEM;
> >>> +     if (!new_bus)  {
> >>> +             pr_err("kvm: failed to shrink bus, removing it completely\n");
> >>> +             goto broken;
> >>
> >> The guest will fail in mysterious ways, if you do this (and
> >> io_bus_unregister_dev can be called during runtime): in-kernel device
> >> accesses will fail with unknown behaviour in the guest.
> 
> Actually, the next access to the BUS should result in -ENOMEM. And the
> error message should be enough to then figure out what went wrong.

Hopefully, an admin will look at the logs :)

But yes, the patch should have caught all issues in the host, and the
guest will basically be presented with broken "hardware".

> However, to hit this scenario at all feels very unlikely. So I would
> like to avoid advanced allocation schemes.

Agreed, spending too much time on complex recovery scenarios is
overkill for this unlikely case.