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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Thu, Sep 14, 2017 at 05:18:44PM +0100, Joao Martins wrote:
> On 09/14/2017 05:10 PM, Wei Liu wrote:
> > On Thu, Sep 07, 2017 at 05:53:54PM +0100, 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;
> >>
> >> - Backend reads this entries and uses <feature-value> as the value of
> >> <feature-name>, which will then be visible on the frontend.
> >>
> >> [ 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"
> > 
> > There needs to be a way to distinguish parameters consumed by toolstack
> > vs the ones passed on to backends. The parameters passed to backends
> > should start with a predefined prefix.
> > 
> Hmm, which seems to be inline with the "request" prefix when controlling 
> certain
> features enabled/disabled? Oh wait, perhaps you mean wrt to the 
> UI/config-format
> rather than xenstore entries and such? If it's the latter, see below,

I was thinking about xl config syntax.

> 
> >> ]
> >> ...
> >>
> >> 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
> >>
> >> 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
> >>
> >> 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.
> >>
> 
> In case the above was about config format, this one suggested above sounds 
> more
> general, and easy to reuse across backends. Maybe instead of "features", could
> be "backend_features" since, most PV backends declare a "backend" and a
> "backend_id" as per libxl IDL.
> 

The proposed syntax looks a bit difficult to parse.

What's wrong with request-XXX=YYY syntax? We can have many of those as
we like. Xl just picks those and concatenate them into backend_features.

Assuming we just dump things into backend_features, once the syntax is
determined, we can only extend it but not change it because we need to
maintain backward-compatibility in both xl and libxl.

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