Re: [Qemu-devel] vhost-user: VHOST_SET_MEM_TABLE, VHOST_SET_VRING_CALL need a reply?

[Michael S. Tsirkin]

On Thu, Sep 18, 2014 at 08:45:37AM +0800, Linhaifeng wrote:
> 
> 
> On 2014/9/17 17:56, Michael S. Tsirkin wrote:
> > On Wed, Sep 17, 2014 at 05:39:04PM +0800, Linhaifeng wrote:
> >> I think maybe is not need for the backend to wait for response.
> >>
> >> There is another way.vhost-user send "VHOST_GET_MEM_TABLE" to qemu then 
> >> qemu send VHOST_SET_MEM_TABLE to update the regions of vhost-user.same as 
> >> other command.
> >> If qemu could response the request of the vhost-user.the vhost-user could 
> >> update date at anytime.
> > 
> > The updates are initiated by QEMU, as a result of IOMMU,
> > memory hotplug or some other configuration change.
> 
> How to deal with the vhost-user restart?
> when vhost-user restart it will lost the infomation which QEMU send.
> 
> In the kernel mode vhost will restart with QEMU but in the user mode vhost 
> will not.

vhost-user must restart with qemu only.

The nature of virtio protocol is such that there's not enough in-memory
state for host to gracefully recover from losing VQ state.

We could add a new feature to allow recovery by reporting
failure to guest, and disabling processing new requests.
Guest could respond by recovering / discarding submitted
requests, re-enabling the device (likely by executing a reset)
and re-submitting requests.


This would need a new feature bit though, and would have to
be acknowledged by guest. As such this would have to be
virtio 1.0 feature, virtio 0.x is frozen.

> > 
> >> I think it's very useful for Commercialization.
> >>
> >> On 2014/9/17 16:38, Michael S. Tsirkin wrote:
> >>> Reply-To: 
> >>>
> >>> Thinking about the vhost-user protocol, VHOST_SET_MEM_TABLE
> >>> is used to update the memory mappings.
> >>>
> >>> So shouldn't we want for response?
> >>> Otherwise e.g. guest can start using the memory
> >>> that vhost-user can't access.
> >>>
> >>> Similarly, with an IOMMU vhost-user might access memory it shouldn't.
> >>>
> >>> VHOST_SET_VRING_CALL is used for MSI-X masking.
> >>> Again, after vector is masted by switching the call fd,
> >>> backend shouldn't assert the old one.
> >>>
> >>> Thoughts?
> >>>
> >>>
> > 
> > .
> >