Re: [Qemu-devel] [virtio-dev] [PATCH v3 0/7] Vhost-pci for inter-VM communication

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

>  * VHOST_USER_SET_VRING_KICK
> 
>    Set up vring kick doorbell (unless bit 8 is set) before sending
>    VHOST_USER_SET_VRING_KICK to the guest.

But guest can't use it, now can it?

What guest needs is a mapping to interrupts.


>  * VHOST_USER_SET_VRING_CALL
> 
>    Set up the vring call doorbell (unless bit 8 is set) before sending
>    VHOST_USER_SET_VRING_CALL to the guest.

Same here. what guest needs is mapping from io to notifications,
right?

---

> > I took a quick look and I doubt we can do something that is both
> > compatible with the existing vhost-user and will make it possible to
> > extend the protocol without qemu changes. Let's assume I pass a new
> > message over the vhost-user channel.  How do we know it's safe to pass
> > it to the guest?
> >
> > That's why we gate any protocol change on a feature bit and must parse
> > all messages.
> 
> QEMU must parse all messages and cannot pass through unknown messages.
> Think of QEMU vhost-pci as both a vhost-user slave to the other VM and
> a vhost-user master to the guest.
> 
> QEMU changes are necessary when the vhost protocol is extended.
> Device interface changes are only necessary if doorbells or shared
> memory regions are added, any other protocol changes do not change the
> device interface.
> 
> Stefan

I guess you have a different definition of a device interface than
myself - I consider it an interface change if a feature bit changes :)

-- 
MST