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Re: [Xen-devel] [ARM] Native application design and discussion (I hope)



Hi Andrii,

On 24/04/2017 17:56, Andrii Anisov wrote:
On 21.04.17 23:58, Stefano Stabellini wrote:
On Fri, 21 Apr 2017, Andrii Anisov wrote:
We will also need another type of application: one which is
periodically called by XEN itself, not actually servicing any domain
request. This is needed for a
coprocessor sharing framework scheduler implementation.
EL0 apps can be a powerful new tool for us to use, but they are not the
solution to everything. This is where I would draw the line: if the
workload needs to be scheduled periodically, then it is not a good fit
for an EL0 app.
From my last conversation with Volodymyr I've got a feeling that notions
"EL0" and "XEN native application" must be pretty orthogonal.
In [1] Volodymyr got no performance gain from changing domain's
exception level from EL1 to EL0.
Only when Volodymyr stripped the domain's context  abstraction (i.e.
dropped GIC context store/restore) some noticeable results were reached.

Do you have numbers for part that take times in the save/restore? You mention GIC and I am a bit surprised you don't mention FPU.

I would have a look at optimizing the context switch path. Some ideas:
- there are a lot of unnecessary isb/dsb. The registers used by the guests only will be synchronized by eret.
        - FPU is taking time to save/restore, you could make it lazy
- It might be possible to limit the number of LRs saved/restored depending on the number of LRs used by a domain.
        - ...

If the numbers are still bad, then we can start stripping some part (for instance you may not need FPU).

Cheers,

--
Julien Grall

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