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Re: [Xen-devel] [XenSummit 2017] Shared coprocessor framework followup

Hello Edgar,

On 01.08.17 20:13, Edgar E. Iglesias wrote:
Are master ports behind IOMMU?
Yes, they are.
What IOMMU IP is used?

It's possible to reprogram the configuration of the PL and swap accelerators in
and out on the fly. It's probably going to be too slow for trying to
context switch between guests
So let us assume it is a FW-less resource we need to share between domains.
Context switch will be stripped to mapping its mmio (or passing mmio
accesses) next domain to serve and IOMMU configuration switching.
Yep, IOMMU matters.
OK. I think the PL is more like a firmware enabled resource.
The PL configuration needs to be loaded much like firmware
otherwise the accelerators can't change shape and all guests
must use the same kind.
I understand this.
But I got your words like you are going to give a try to the same kind for all domains first. Because you assumed that reconfiguring would be too slow, what is actually discussable.

  so I think primarily we would be looking at
a way to lets say, "allocate" and "release" the resources for batch use.
Kind of voluntary preemption?
Right. That could be a start.
In the future perhaps it makes sense to context-switch.
We still need the context switch to be done. The difference is that now it could be done only when the accelerator is not busy.

If a guest cannot allocate an accelerator, it could fall back to emulation
or just to using SW libraries until an accelerator slot is available.
What about the thing I called "an access emulation" [1]? From the domain's
point of view it would be reflected in a delayed response (via IRQ or
register polling) from an accelerator.

I guess the concept described above is feasible even with current SCF code
and will not take too much efforts.
I'll have a look, thanks!
Do not hesitate to contact us in case you need any help or clarification.


*Andrii Anisov*

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