linux-kernel - Re: virtio config

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Message-ID: <4730F351.5060006@us.ibm.com>
Date:	Tue, 06 Nov 2007 17:05:53 -0600
From:	Anthony Liguori <aliguori@...ibm.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	Dor Laor <dor.laor@...ranet.com>, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: virtio config_ops refactoring

Rusty Russell wrote:
> On Wednesday 07 November 2007 04:48:35 Anthony Liguori wrote:
>> Semantically, find requires that a field have both a type and a length.
>> With the exception of the VIRTQUEUE field used internally by lguest,
>> type is always a unique identifier.  Since virtqueue information is not
>> a required part of the config space, it seems to me that type really
>> should be treated as a unique identifier.
> 
> Hi Anthony,
> 
> 	Not sure I get this.  It is a unique identifier.  You need the length
> to handle unknown fields.

It's not a unique identifier since it can be used for multiple items 
(like it is for virtqueues configs).

>> find_vq also is curious in that it is stateful in it's enumeration.
> 
> Well, they're *all* stateful.  This gives a simple method of knowing what 
> fields the guest understands: it marks the fields as it finds them.  Then it 
> sets the status, which allows the host to know when it's completed 
> configuration reads.

But PCI device configuration is not stateful.  If you care about letting 
the host know what features a guest understands, I think something more 
explicit and stateful should be used.  For instance, a feature register 
that stores a bitmap.

Otherwise, the host has to infer based on what fields that guest has 
read what features the guest actually supports.  That seems error prone 
to me.

> I like enumerating the virtqueues: it's not necessary but it's clearer.
> 
>> This adds seemingly unnecessary complexity.
> 
> I'd be happy for a simpler mechanism...

What do you think of what I proposed?  It seems simpler to me.

Regards,

Anthony Liguori

> Cheers,
> Rusty.

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