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Re: [Qemu-devel] device assignment for embedded Power


From: Alexander Graf
Subject: Re: [Qemu-devel] device assignment for embedded Power
Date: Sat, 2 Jul 2011 04:17:24 +0200

On 02.07.2011, at 01:50, Paul Brook wrote:

>> On Fri, 2011-07-01 at 21:59 +0100, Paul Brook wrote:
>>>> On Fri, 1 Jul 2011 18:03:01 +0100
>>>> 
>>>> Paul Brook <address@hidden> wrote:
>>>>> Basically you should start by implementing full emulation of a device
>>>>> with similar characteristics to the one you want to passthrough.
>>>> 
>>>> That's not going to happen.
>>> 
>>> Why is your device so unique? How does it interact with the guest system
>>> and what features does it require that doen't exist in any device that
>>> can be emulated?
>> 
>> Do you guys only support PCI pass-through by doing full emulation of the
>> all possible supported PCI devices first ? :-)
> 
> Absolutely not.  My point is that dynamic (user-driven) device creation is 
> effectively a prerequisite for a passthrough device.
> 
> If you just want to make a very specific use-case then this doesn't need any 
> code in qemu at all.  We just make the user provide the device tree 
> themselves. If it doesn't match then they loose.  If you do choose an ugly 
> qemu then the changes are it'll be changed/removed once we do dyamic device 
> creation properly.  There have already been discussions about dynamic device 
> creation, so this this isn't completely hypothetical.
> 
> If you integrate it properly, then you need to realise then there's a fair 
> chunk of infrastructure and user interface required.  Most of which has 
> nothing to do with device passthrough.  Trying to implement both at the same 
> time is just going to cause confusion and complicate things.  It's already a 
> hard problem, combining it with something else is just going to cause you and 
> everyone else even more pain.

So you're basically saying we should tackle these 3 issues separately:

  * actually pass through a device
  * generate interrupt links
  * model the guest device tree dynamically based on whatever the user gives us

I tend to agree with that perspective. Still, the main issue still stands in 
that we don't have a concrete answer for all three issues :). Facing them one 
at a time might help actually solving them though.


Alex




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