On Thu, Jul 30, 2015 at 09:31:34AM +0200, Igor Mammedov wrote:
> On Thu, 30 Jul 2015 09:29:56 +0300
> "Michael S. Tsirkin" <m...@redhat.com> wrote:
>
> > On Thu, Jul 30, 2015 at 09:25:55AM +0300, Michael S. Tsirkin wrote:
> > > On Thu, Jul 30, 2015 at 08:22:18AM +0200, Igor Mammedov wrote:
> > > > On Wed, 29 Jul 2015 18:03:36 +0300
> > > > "Michael S. Tsirkin" <m...@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).
> > >
> > > Where exactly does it fail?
> > > memory_listener_register returns void so clearly it's not that ...
> >
> > Oh, dev_start fails. But that's not called at device_add time.
> > And vhost-user can't fall back to anything.
> Yes, looks like it would lead to non functional vhost-user backed device
> since there isn't any error handling at that stage.
>
> But it's would be the same without memory hotplug also, one just has to
> start QEMU with several -name memdev=xxx options to cause that condition.
Absolutely. And kvm has this problem too if using kernels before 2014.
But I have a question: do we have to figure the number of
chunks exactly? How about being blunt, and just limiting the
number of memory devices?
How about this:
- teach memory listeners about a new "max mem devices" field
- when registering a listener, check that # of mem devices
does not exceed this limit, if not - fail registering
listener
- when adding mem device, check no existing listener
has a limit that conflicts with it
Of course we could add a separate linked list+ register API with just this
field instead of adding it to a memory listener, if that seems
more appropriate.
> Probably the best place to add this check is at vhost_net_init()
> so that backend creation fails when one tries to add it on monitor/CLI
I'd say vhost_dev_init - it's not network specific at all.
> >
> >
> > > > >
> > > > >
> > > > > >
> > > > > > 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
> >
