Re: Notes on stubdoms and latency on ARM

[George Dunlap <george.dunlap@citrix.com>]

On 19/05/17 20:45, Volodymyr Babchuk wrote:
> Hi Stefano,
> 
> On 18 May 2017 at 22:00, Stefano Stabellini <sstabellini@kernel.org> wrote:
> 
>> Description of the problem: need for a place to run emulators and
>> mediators outside of Xen, with low latency.
>>
>> Explanation of what EL0 apps are. What should be their interface with
>> Xen? Could the interface be the regular hypercall interface? In that
>> case, what's the benefit compared to stubdoms?
> I imagined this as separate syscall interface (with finer policy
> rules). But this can be discussed, of course.

I think that's a natural place to start.  But then you start thinking
about the details: this thing needs to be able to manage its own address
space, send and receive event channels / interrupts, &c &c -- and it
actually ends up looking exactly like a subset of what a stubdomain can
already do.

In which case -- why invent a new interface, instead of just reusing the
existing one?

>> The problem with stubdoms is latency and scheduling. It is not
>> deterministic. We could easily improve the null scheduler to introduce
>> some sort of non-preemptive scheduling of stubdoms on the same pcpus of
>> the guest vcpus. It would still require manually pinning vcpus to pcpus.
> I see couple of other problems with stubdoms. For example, we need
> mechanism to load mediator stubdom before dom0.

There are a couple of options here.  You could do something like the
Xoar project [1] did, and have Xen boot a special-purpose "system
builder" domain, which would start both the mediator and then a dom0.
Or you could have a mechanism for passing more than one domain / initrd
to Xen, and pass Xen both the mediator stubdom as well as the kernel for
dom0.

[1] tjd.phlegethon.org/words/sosp11-xoar.pdf

>> Then, we could add a sched_op hypercall to let the schedulers know that
>> a stubdom is tied to a specific guest domain.
> What if one stubdom serves multiple domains? This is TEE use case.

Then you don't make that hypercall. :-)  In any case you certainly can't
use an EL0 app for that, at least the way we've been describing it.

 -George


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