Re: [Embedded-pv-devel] [Xen-devel] [RFC] Shared coprocessor framework

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 23 Nov 2016, Andrii Anisov wrote:
> Stefano,
> 
> Please see my answers below:
> >
> > Thank you for the document, I think is a very good start. I also see the
> > need for this framework. Please add more details about the proposed
> > interface (Xen API, hypercalls, etc) in the next version; I am looking
> > forward to it.
> 
> We will come up with the document update once we have it agreed internally.
> 
> >> AA> Due to the fact that some domain assigned a vcoproc could access 
> >> coproc when
> >> AA> it is running another domain context, framework will implement iomem 
> >> access
> >> AA> emulation for domains which are not provided coproc at the moment of 
> >> access.
> >
> > This is certainly going to be the hardest part. I take the framework is
> > just going to provide a generic API for implementing a coprocessor
> > emulator and it is going to be up to each coprocessor implementation to
> > provide the code.
> 
> This piece together with the context switching logic is definitely a
> platform specific stuff and its complexity could be different
> coprocessor to coprocessor.
> Registers access emultaion for not running vcopro is expected to be
> not very complex for our case:
>     - saved context return on register read
>     - stacking on writes to be executed during switching context to that 
> vcoproc
>     - rare more complex corner cases
> 
> > Is the emulator going to live in the Xen hypervisor?
> That is the idea.
> 
> > It would be nice to provide a simple coprocessor example, if you have one.
> I'm not really sure about a simple functional example. We are
> targeting GPU sharing for the first drop.
 
I was thinking of something trivial but enough to prove the point.
Something like a very simple accelerator, maybe a data copy accelerator.
A GPU is certainly not trivial :-)

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