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

[Paolo Bonzini]

On 19/04/2018 17:59, Michael S. Tsirkin wrote:
> On Thu, Apr 19, 2018 at 05:51:51PM +0200, Paolo Bonzini wrote:
>> On 19/04/2018 17:19, Michael S. Tsirkin wrote:
>>>> - 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.
>>
>> Yes, you can, exactly because some features are defined not by the
>> machine type but rather by the host kernel.  See virtio_net_get_features
>> in QEMU's hw/virtio/virtio-net.c, and virtio_set_features_nocheck in
>> QEMU's hw/virtio/virtio.c.
> 
> Oh you are right. Well we can just special-case that one :)

Anything wrong with bumping the PCI revision?

Paolo