linux-kernel - Re: Use of virtio device IDs

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Message-ID: <4732AE9A.4070701@qumranet.com>
Date:	Thu, 08 Nov 2007 08:37:14 +0200
From:	Avi Kivity <avi@...ranet.com>
To:	Gregory Haskins <gregory.haskins.ml@...il.com>
CC:	Anthony Liguori <aliguori@...ibm.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Dor Laor <dor.laor@...ranet.com>,
	virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: Use of virtio device IDs

Gregory Haskins wrote:
>
>> PCI means that you can reuse all of the platform's infrastructure for
>> irq allocation, discovery, device hotplug, and management.
>>     
>
> Its tempting to use, yes.  However, most of that infrastructure is
> completely inappropriate for a PV implementation, IMHO.  

Why?

> You are
> probably better off designing something that is PV specific instead of
> shoehorning it in to fit a different model (at least for the things I
> have in mind).  

Well, if we design our pv devices to look like hardware, they will fit
quite well.  Both to the guest OS and to user's expectations.

> Its not a heck of a lot of code to write a pv-centric
> version of these facilities.
>
>   

It is.  Especially if you consider Windows and a gazillion versions of
deployed, non-pv-capable Linux systems.  For pv-friendly newer Linux,
it's probably doable, but why?

Look at the mess Xen finds itself in.

>> You can write it for new guests but backporting it to older guests will be a
>> huge task.
>>
>> We will support non-pci for s390, but in order to support Windows and
>> older Linux PCI is necessary.
>>     
>
> I don't know if I would agree with "necessary".  "Easier" perhaps. ;) By
> definition once you are PV you are hypervisor aware.  Now its just a
> matter of plugging in the appropriate plumbing to bridge the hypervisor
> to the guest-os.  Some might be easier than others, sure.  But all
> should be extensible to a degree.
>
>   

It's "necessary" in a pragmatic sense: we want to deliver drivers that
provide features for a wide variety of guests in a reasonable
timeframe.  And that means no rewriting guest OS infrastructure.


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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