Re: [Qemu-devel] [PATCH for-2.4 v2 0/2] vhost: check if vhost has capacity for hotplugged memory

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 29 Jul 2015 18:03:36 +0300
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Wed, Jul 29, 2015 at 01:49:47PM +0200, Igor Mammedov wrote:
> > v1->v2:
> >   * replace probbing with checking for
> >     /sys/module/vhost/parameters/max_mem_regions and
> >     if it's missing has non wrong value return
> >     hardcoded legacy limit (64 slots).
> > 
> > it's defensive patchset which helps to avoid QEMU crashing
> > at memory hotplug time by checking that vhost has free capacity
> > for an additional memory slot.
> 
> What if vhost is added after memory hotplug? Don't you need
> to check that as well?
vhost device can be hotplugged after memory hotplug as far as
current slots count doesn't exceed its limit,
if limit is exceeded device_add would fail or virtio device
would fallback to non vhost mode at its start-up (depends on
how particular device treats vhost_start failure).

> 
> 
> > 
> > Igor Mammedov (2):
> >   vhost: add vhost_has_free_slot() interface
> >   pc-dimm: add vhost slots limit check before commiting to hotplug
> > 
> >  hw/mem/pc-dimm.c                  |  7 +++++++
> >  hw/virtio/vhost-backend.c         | 21 ++++++++++++++++++++-
> >  hw/virtio/vhost-user.c            |  8 +++++++-
> >  hw/virtio/vhost.c                 | 21 +++++++++++++++++++++
> >  include/hw/virtio/vhost-backend.h |  2 ++
> >  include/hw/virtio/vhost.h         |  1 +
> >  stubs/Makefile.objs               |  1 +
> >  stubs/vhost.c                     |  6 ++++++
> >  8 files changed, 65 insertions(+), 2 deletions(-)
> >  create mode 100644 stubs/vhost.c
> > 
> > -- 
> > 1.8.3.1