Re: [Xen-devel] Feature control on PV devices

[Joao Martins <joao.m.martins@xxxxxxxxxx>]

[ Realized that I didn't CC the maintainers,
  so doing that now, +Linux folks +PV interfaces czar
  Sorry for the noise! ]

On 09/08/2017 09:49 AM, Joao Martins wrote:
> [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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