RE: Virtio BoF minutes from KVM Forum 2017

[Ilya Lesokhin <ilyal@mellanox.com>]

On Wednesday, November 1, 2017 7:35 PM, Michael S. Tsirkin wrote:
> > You have to either use and additional descriptor for metadata per chain.
> > Or putting it in one of the buffers and forcing the lifetime of the metadata
> and data to be the same.
> 
> That's true. It would be easy to make descriptors e.g. 32 bytes each, so
> you can add extra data in there. Or if we can live with wasting some
> bytes per descriptor, we could add a descriptor flag that marks the
> address field as meta-data. You could then chain it with a regular
> descriptor for data. However all in all the simplest option is probably
> in the virtio header which can be linear with the packet.
[I.L] In the current proposal descriptor size == SGE (scatter gather entry) size.
I'm not sure that's a good idea.
For example we are considering having an RX ring were you just post a list
of PFN's so a sge is only 8 bytes.

I might be wrong here, so please correct me if that's the not the case.
but I've gotten the impression that due to DPDK limitations you've focused 
on the use case where you have 1 SGE.
I'm not sure that a representative workload for network devices,
As LSO is an important offload.

And the storage guys also complained about this issue.


> I suspect a good way to do this would be to just pass offsets within the
> buffer back and forth. I agree sticking such small messages in a
> separate buffer is not ideal. How about an option of replacing PA
> with this data?
[I.L] PA?

> 
> 
> 
> > 3. There is a usage model where you have multiple produce rings
> > And a single completion ring.
> 
> What good is it though? It seems to perform worse than combining
> producer and consumer in my testing.
> 
[I.L] It might be that for virtio-net a single ring is better
But are you really going to argue that its better in all possible use cases?

> 
> > You could implement the completion ring using an additional virtio ring,
> but
> > The current model will require an extra indirection as it force you to write
> into
> > The buffers the descriptor in the completion ring point to. Rather than
> writing the
> > Completion into the ring itself.
> > Additionally the device is still required to write to the original producer ring
> > in addition to the completion ring.
> >
> > I think the best and most flexible design is to have variable size descriptor
> that
> > start with a dword header.
> > The dword header will include - an ownership bit, an opcode and
> descriptor length.
> > The opcode and the "length" dwords following the header will be device
> specific.
> 
> This means that device needs to do two reads just to decode the
> descriptor fully. This conflicts with feedback Intel has been giving on
> list which is to try and reduce number of reads. With header linear with
> the packet, you need two reads to start transmitting the packet.
> 
[I.L] The device can do a single large read and do the parsing afterword's.
You could also use the doorbell to tell the device how much to read.


> Seems to look like the avail bit in the kvm forum presentation.
> 	
[I.L] I don't want to argue over the name. The main difference in my 
proposal is that the device doesn't need to write to the descriptor.
If it wants to you can define a separate bit for that.

> > Each device (or device class) can choose whether completions are reported
> directly inside
> > the descriptors in that ring or in a separate completion ring.
> >
> > completions rings can be implemented in an efficient manner with this
> design.
> > The driver will initialize a dedicated completion ring with empty completion
> sized descriptors.
> > And the device will write the completions directly into the ring.
> 
> I assume when you say completion you mean used entries, if I'm wrong
> please correct me.  In fact with the proposal in the kvm forum
> presentation it is possible to write used entries in a separate address
> as opposed to overlapping the available entries.  If you are going to
> support skipping writing back some used descriptors then accounting
> would have to change slightly since driver won't be able to reset used
> flags then.  But in the past in all tests I've written this separate
> ring underperforms a shared ring.
> 
[I.L] A completion is a used entry + device specific metadata.
I don't remember seeing an option to write used entries in a separate address,
I'll appreciate it if you can point me to the right direction.

Regarding the shared ring vs separate ring, I can't really argue with you
as I haven't done the relevant measurement.
I'm just saying it might not always be optimal in all use case,
So you should consider leaving both options open.

Its entirely possible that for virtio-net you want a single ring,
Where for PV-RDMA you want separate rings.