Re: [Xen-devel] [RFC] scf: SCF device tree and configuration documentation

[Ian Jackson <ian.jackson@xxxxxxxxxxxxx>]

Andrii Anisov writes ("Re: [RFC] scf: SCF device tree and configuration 
documentation"):
> On 05.05.17 17:13, Ian Jackson wrote:
> > If these regions of the DT can be marked by this "xen,coproc"
> > property, can't we instead identify them (eg in the libxl domain
> > configuration) by their DT path ?  So then you could say "please pass
> > through coprocessor /aliases/soc/coproc0" or something.
>
> Yep, exactly this approach worked for us when there were no requirement 
> to spawn for one guest domain several vcoprocs from one physical 
> coprocessor.
> That requirement leads to a need of mapping "second" vcoproc mmio ranges 
> to different addresses and potentially using another IRQ, in order to 
> let a domain treat those devices separately.

Why wouldn't the toolstack simply choose appropriate irqs/mmio
ranges ?  I would expect the virtual irqs/mmio ranges to not
necessarily match the physical ones anyway.  Is choosing these ranges
complicated ?

> > Also, the proposal there does not seem to provide any way to say which
> > guest should get any particular coprocessor.
> No, its not about getting (owning) a coprocessor for some domain, but 
> get a virtual coprocessor (vcoproc).
> Actually a vcoproc abstraction is inspired by vcpu abstraction, maybe 
> such view would help to get an idea.
> Please also refer [1] to get the high level overview of the topic.

Thanks.  I still feel like I am labouring under some misapprehensions.
I hope you'll clear them up as they become evident...

What I mean is that your previous proposal doesn't provide any way to
say which guest(s) should get instances of this virtual coprocessor.
Some guests should get none; some one; etc.

Also, I am perplexed by your suggestion that a single physical coproc
might be presented to a guest as two vcoprocs.  If your sharing
strategy is context-switching, is this not going to result in a lot of
context-switching, whenever the guest (which thinks it has two
coprocs) touches one and then the other ?

Obviously there are many things I still don't understand here.

Regards,
Ian.

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