[virt-tools-list] vhostmd - virtio channel support

[Daniel P. Berrangé]

On Fri, Jun 08, 2018 at 12:01:48PM -0600, Jim Fehlig wrote:
> On 06/08/2018 02:30 AM, Daniel P. Berrangé wrote:
> > On Thu, Jun 07, 2018 at 04:23:32PM -0600, Jim Fehlig wrote:
> > > On 06/07/2018 09:42 AM, Trapp, Michael wrote:
> > > > I fully understand the requirements regarding coding style and the way the changes should be split and discussed in detail,
> > > > but may I ask you to provide some general feedback about the basic concept and if this an option at all before we start discussing the details.
> > > 
> > > vhostmd already supports 'vbd' and 'xenstore' as transports for the metric
> > > data. Adding another virtio-channel based transport is a great idea IMO.
> > > 
> > > WRT writing the data to the various channels, I suppose it is possible to do
> > > that even if there is no (immediate) recipient for the data? The vbd and
> > > xenstore transports don't have a protocol so to speak. The data is simply
> > > written to the disk or xenstore node regardless if there is someone to
> > > consume it. Can the virtio transport work the same? If so, that would
> > > alleviate the need for polling all the fds and the static list of
> > > epoll_event. In virtio_metrics_update() we could simply connect to all the
> > > channels, write the data, close them, and repeat on the next update period
> > > (which is every 5sec by default).
> > 
> > Why push it to the guest at a fixed frequency, rather than just letting the
> > guest query the host at whatever rate it feels it needs ?
> 
> The other transports work that way, but in those cases the guests use the
> same source on the host, e.g. a ramdisk for vbd. For virtio where the source
> is a per-guest channel on the host, I suppose it's best to only send the
> metrics when the guest asks for them.

Yeah I tend to view this use case as being the reverse of QEMU guest agent,
rather it is a QEMU host agent. So the guest is the client making calls to
a daemon in the host to get info.

Regards,
Daniel
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