linux-kernel - Re: Using DT overlays for adding virtual hardware

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Date:	Wed, 8 Jun 2016 19:31:34 +0300
From:	Pantelis Antoniou <pantelis.antoniou@...sulko.com>
To:	Mark Rutland <mark.rutland@....com>
Cc:	Jan Kiszka <jan.kiszka@...mens.com>,
	devicetree <devicetree@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jailhouse <jailhouse-dev@...glegroups.com>,
	Måns Rullgård <mans@...x.de>,
	Antonios Motakis <antonios.motakis@...wei.com>
Subject: Re: Using DT overlays for adding virtual hardware

Hi Mark,

> On Jun 8, 2016, at 19:23 , Mark Rutland <mark.rutland@....com> wrote:
> 
> On Wed, Jun 08, 2016 at 06:57:37PM +0300, Pantelis Antoniou wrote:
>> Hi Mark,
>> 
>>> On Jun 8, 2016, at 18:17 , Mark Rutland <mark.rutland@....com> wrote:
>>> 
>>> On Wed, Jun 08, 2016 at 04:16:32PM +0200, Jan Kiszka wrote:
>>>> Hi all,
>>>> 
>>>> already started the discussion off-list with Pantelis, but it's better
>>>> done in public:
>>>> 
>>>> I'm currently exploring ways to make Linux recognize dynamically added
>>>> virtual hardware when running under the Jailhouse hypervisor [1]. We
>>>> need to load drivers for inter-partition communication devices that only
>>>> appear after Jailhouse started (which is done from within Linux, i.e.
>>>> long after boot) or when a partition was added later on. Probably, we
>>>> will simply add a virtual PCI host bridge on systems without physical
>>>> PCI and let the IPC device be explored that way (already works on x86).
>>>> Still, that leaves us with hotplug and unplug on hypervisor activation
>>>> and deactivation.
>>> 
>>> If I've understood correctly you want to use overlays to inject the
>>> virtual PCI host bridge?
>>> 
>>> Given that you know precisely what you want to inject, I'm not sure I
>>> see the value of using an overlay. 
>>> 
>>> Is there some reason you can't just create a device without having to go
>>> via an intermediate step? As I understand it, Xen does that for (some)
>>> virtual devices provided to Dom0 and DomU.
>> 
>> As far as I understand it PCI is just one of the cases. You could conceivably
>> inject any kind of virtio device like serial/storage networking etc.
> 
> Sure, but we already have PCI transport for virtio devices, and per the
> above PCI is the transport used on x86, so I assume that the devices we
> really care about are going to be PCI anyhow.
> 

PCI on VMs is a hack, it’s all emulated.

We’re using it as crutch because it’s ubiquitous and is capable
of probing, but it comes with a considerable amount of baggage.
Jailhouse is a particular kind of a hypervisor where it is intended for
safety critical applications and designed to be certified as such.
The less amount of code it contains the better, and much easier to certify. 

>> The question is since overlays exist and do work, why should he do anything else
>> besides using them?
> 
> For one thing, they only work with DT, and there are ACPI ARM server
> platforms out there, for which people may wish to use jailhouse. Tying
> this to DT is not necessarily the best idea.
> 

I just don’t see how an ACPI based hypervisor can ever be certified for
safety critical applications. It might be possible but it should be
an enormous undertaking; perhaps a subset without AML, but then again
can you even boot an ACPI box without it?

DT is safer since it contains state only.

> To be clear, I'm not arguing *against* overlays as such, just making
> sure that we're not prematurely choosing a solution just becasue it's
> the one we're aware of.
> 
> Thanks,
> Mark.

Regards

— Pantelis