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

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Message-Id: <28BC1AEC-6A7D-4A97-82F4-5670E884C41D@konsulko.com>
Date:	Wed, 8 Jun 2016 18:57:37 +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 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.

The question is since overlays exist and do work, why should he do anything else
besides using them? I doubt he’s interested in writing yet another driver for
something that already exists and works.

> Thanks,
> Mark.

Regards

— Pantelis