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Subject: Re: Virtio BoF minutes from KVM Forum 2017

On Sun, Oct 29, 2017 at 01:52:25PM +0100, Jens Freimann wrote:
> Ilya: - you might have more completions than descriptors available
> - partial descriptor chains are a problem for hardware because you might have
>  to read a bunch of conscriptors twice - how would you do deal with a big
> buffer that cointains a large number of
>  small packets with respect to completions?
> - is one bit for completion enough? right now it means descriptor was actually
>  used. how to we signal when it was completed?

I am not sure I understand the difference. Under virtio, driver makes a
descriptor available, then device reads/writes memory depending on
descriptor type, then marks it as used.

What does completed mean?

> - concerned about not being able to do scatter/gatter with the ring layout.
>  Network drivers heavily using indirect buffers.  - for a hardware
> implementation a completion ring is a very convenient form for
>  some use cases, so we want an efficient implementation for them. If we had an
>  inline descriptor then a completion ring is just a normal ring and we won't
>  need another ring type.
> - doesn't like the fact that we need to do a linear scan to find the length of
>  a descriptor chain. It would be nice if we could have the length of the chain
>  in the first descriptor (i.e. the number of chained descriptors, not the number
>  of posted descriptors which can be deduced from the id field)

Not responding to rest of points since I don't understand the basic
assumption above yet.


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