Re: virtio message framing

[Anthony Liguori <aliguori@us.ibm.com>]

On 04/13/2012 09:50 AM, Stefan Hajnoczi wrote:
> The virtio specification says:
>
> "The descriptors used for a buff^[er should not eff^[ect the semantics
> of the message,
> except for the total length of the bu^[ffer"
>
> and
>
> "In particular, no implementation should use the descriptor boundaries
> to determine the size of any header in a request"

This was the noble intention but all of the implementations actually rely on 
boundary sizes.

Both QEMU and lguest rely on boundary sizes.  We've removed some of it in 
virtio-net in QEMU but it still looks like it's there for virtio-blk.

kvm tool also makes this assumption.

> Why should descriptor layout not be specified?
>
> It seems that implementing arbitrary descriptor layout support (e.g.
> 1-byte descriptors) requires more code and makes input validation
> harder.
>
> Why bother with the flexibility of unspecified descriptor layouts?  As
> long as the layout is specified clearly it makes everyone's lives
> easier to use a strict descriptor layout.

I hate to just change the spec here but I don't see a better option.

Regards,

Anthony Liguori

>
> The only reason I can think of is that virtio should work over
> transports that do not have the concept of "descriptors" (non-vring
> transports like pipes or streams).
>
> Stefan
>