Re: [Qemu-devel] [virtio-dev] RE: [PATCH v3 6/6] vhost-user: support registering external host notifiers

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Apr 19, 2018 at 03:02:40PM +0200, Paolo Bonzini wrote:
> On 19/04/2018 14:43, Liang, Cunming wrote:
> >> 2. Memory barriers. Right now after updating the avail idx, 
> >> virtio does smp_wmb() and then the MMIO write. Normal hardware
> >> drivers do wmb() which is an sfence. Can a PCI device read bypass
> >> index write and see a stale index value?
> >
> > A compiler barrier is enough on strongly-ordered memory platform. As
> > it doesn't re-order store, PCI device won't see a stale index value.
> > But a weakly-ordered memory needs sfence.
> 
> That is complicated then.  We need to define a feature bit and (in the
> Linux driver) propagate it to vring_create_virtqueue's weak_barrier
> argument.  However:
> 
> - if we make it 1 when weak barriers are needed, the device also needs
> to nack feature negotiation (not allow setting the FEATURES_OK) if the
> bit is not set by the driver.
>  However, that is not enough.  Live
> migration assumes that it is okay to migrate a virtual machine from a
> source that doesn't support a feature to a destination that supports it.
>  In this case, it would assume that it is okay to migrate from software
> virtio to hardware virtio.  This is wrong because the destination would
> use weak barriers

You can't migrate between systems with different sets of device features
right now.

> - if we make it 1 when strong barriers are enough, software virtio
> devices needs to be updated to expose the bit.  This works, including
> live migration, but updated drivers will now go slower when run against
> an old device that doesn't know the feature bit.
> 
> Maybe bump the PCI revision, so that only the new revision has the bit?
> 
> Thanks,
> 
> Paolo

As a first step, if you want to migrate to a HW offloaded solution
then you need to enable the feature. It does mean it will go
a bit slower when run with software, so it's only good if
most systems in your cluster do have the HW offload.
I think we can start by getting that working and
think about ways to improve down the road.


That's the usecase we designed FEATURES_OK for though, so I do
think/hope it's enough and we don't need to play with revisions.


-- 
MST