Re: Feature control on PV devices

[Joao Martins <joao.m.martins@oracle.com>]

[Forgot two important details regarding Xenbus states]

On 09/07/2017 05:53 PM, Joao Martins wrote:
> Hey!
> 
> We wanted to brought up this small proposal regarding the lack of
> parameterization on PV devices on Xen.
> 
> Currently users don't have a way for enforce and control what
> features/queues/etc the backend provides. So far there's only global parameters
> on backends, and specs do not mention anything in this regard.
> 
> The most obvious example is netback/blkback max_queues module parameter where it
> sets the limit the maximum queues for all devices which is not that flexible.
> Other examples include controlling offloads visible by the NIC (e.g. disabling
> checksum offload, disabling scather-gather), others more about I/O path (e.g.
> disable blkif indirect descriptors, limit number of pages for the ring), or less
> grant usage by minimizing number of queues/descriptors.
> 
> Of course there could be more examples, as this seems to be ortoghonal to the
> kinds of PV backends we have. And seems like all features appear to be published
> on the same xenbus state?
> 
> The idea to address this would be very simple:
> 
> - Toolstack when initializing device paths, writes additional entries in the
> form of 'request-<feature-name>' = <feature-value>. These entries are only
> visible by the backend and toolstack;
>
And after that we switch the device state to XenbusStateInitialising as usual.

> 
> - Backend reads this entries and uses <feature-value> as the value of
> <feature-name>, which will then be visible on the frontend.
> 
And after that we switch state to XenbusStateInitWait as usual. No changes are
involved in xenbus state changes other than reading what the toolstack had
written in "request-*" and seed accordingly. Backends without support would
simply ignore these new entries.

> [ Removal of the 'request-*' xenstore entries could represent a feedback look
>   that the backend indeed read and used the value. Or else it could simply be
>   ignored. ]
> 
> And that's it.
> 
> In pratice user would do: E.g.
> 
> domain.cfg:
> ...
> name = "guest"
> kernel = "bzImage"
> vif = ["bridge=br0,queues=2"]
> disk = [
> "format=raw,vdev=hda,access=rw,backendtype=phy,target=/dev/HostVG/XenGuest2,queues=1,max-ring-page-order=0"
> ]
> ...
> 
> Toolstack writes:
> 
> /local/domain/0/backend/vif/8/0/request-multi-queue-max-queues = 2
> /local/domain/0/backend/vbd/8/51713/request-multi-queue-max-queues = 2
> /local/domain/0/backend/vbd/8/51713/request-max-ring-page-order = 0

/local/domain/0/backend/vbd/8/51713/state = 1 (XenbusStateInitialising)

> 
> Backends reads and seeds with (and assuming it passes backend validation ofc):
> 
> /local/domain/0/backend/vif/8/0/multi-queue-max-queues = 2
> /local/domain/0/backend/vbd/8/51713/multi-queue-max-queues = 2
> /local/domain/0/backend/vbd/8/51713/max-ring-page-order = 0
> 
/local/domain/0/backend/vbd/8/51713/state = 2 (XenbusStateInitWait)

> The XL configuration entry for controlling these tunable are just examples it's
> not clear the general preference for this. An alternative could be:
> 
> vif = ["bridge=br0,features=queues:2\\;max-ring-page-order:0"]
> 
> Which lets us have more generic feature control, without sticking to particular
> features names.
> 
> Naturally libvirt could be a consumer of this (as it already has the 'queues'
> and host 'tso4', 'tso6', etc in their XML schemas)
> 
> Thoughts? Do folks think the correct way of handling this?
> 
> Cheers,
> Joao
> 
> [0] https://github.com/qemu/qemu/blob/master/hw/net/virtio-net.c#L2102
> 

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