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Re: [Qemu-devel] [PATCH RFC 0/3] qdev: order devices by priority before

From: Marcel Apfelbaum
Subject: Re: [Qemu-devel] [PATCH RFC 0/3] qdev: order devices by priority before creating them
Date: Tue, 10 May 2016 14:05:20 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.5.0

On 05/10/2016 11:28 AM, Markus Armbruster wrote:
Marcel Apfelbaum <address@hidden> writes:

This series aims to allow more devices to be used with '-device'
by sorting the devices based on a predefined creation order flag
before creating them.

Devices like IOMMU need to be created before others, so they can leverage
the DeviceCreationPriority flag introduced by the first patch to DeviceClass.

The second patch sorts the devices by their DeviceCreationPriority
before creating them.

Finally, the last patch demonstrates how it can be used to ensure
the creation of host-bridges before the pci-bridges and pci-bridges before
the others.

I preferred to combine all the priorities into a single enum
to better manage the creation order.

This is an RFC because I only wanted to know if it seems like the right way to 
Comments are appreciated,

Hi Markus,
Thanks for looking into this.

Can you explain why requiring the user to specify -device in a sane
order isn't good enough?

Point taken, the truth is I didn't like the 'order' restriction in the first 

If the device creation depends on the id of some other devices (e.g we need the 
bus id to plug a device into it),
for IOMMU devices it gets a little tricky. You can add the IOMMU device before 
other PCI devices but it will not
work (because some internal implementation). This is why we added using 
-machine pc,iommu=on.
I suppose we have other examples as well. This is not user friendly IMO.

To solve the specific IOMMU problem we can check that there are no PCI devices 
created yet,
but I am not sure is a better approach and is strictly related to this device.

The goal is to be able to add more devices with -device and I thought this kind 
of creation in steps may help.


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