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SubjectRe: 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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