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

From: Volodymyr Babchuk <vlad.babchuk@gmail.com>
To: Julien Grall <julien.grall@arm.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>,
	Andrii Anisov <andrii_anisov@epam.com>,
	Dario Faggioli <dario.faggioli@citrix.com>,
	george.dunlap@citrix.com, Xen Devel <xen-devel@lists.xen.org>,
	nd@arm.com, Artem Mygaiev <joculator@gmail.com>
Subject: Re: [ARM] Native application design and discussion (I hope)
Date: Tue, 25 Apr 2017 00:41:16 +0300	[thread overview]
Message-ID: <CAOcqxo3CwXVr9FRa10yTTkibcJNp6AB4_1MgXHwWZdjEYL86LA@mail.gmail.com> (raw)
In-Reply-To: <64e12ab3-92cc-2168-7ba6-975a93adba93@arm.com>

Julien,

>>>> 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 did it in the other thread. Check out [1]. The most speed up I got
after removing vGIC context handling

> 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.
I have removed (almost) all of them. No significant changes in latency.

>         - FPU is taking time to save/restore, you could make it lazy
This also does not takes much time.

>         - It might be possible to limit the number of LRs saved/restored
> depending on the number of LRs used by a domain.
Excuse me, what is LR in this context?

You can take a look at my context switch routines at [2].

[1] http://marc.info/?l=xen-devel&m=149088856116797&w=2
[2] https://github.com/lorc/xen/blob/el0_app/xen/arch/arm/domain.c#L257

-- 
WBR Volodymyr Babchuk aka lorc [+380976646013]
mailto: vlad.babchuk@gmail.com

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