RE: Virtio BoF minutes from KVM Forum 2017

[Ilya Lesokhin <ilyal@mellanox.com>]

On Wednesday, November 01, 2017 4:59 PM, Michael S. Tsirkin wrote:

> 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?
> 

During the BOF, someone raised the point that there is no indication that the HW has
Read the descriptor. I think after some discussion we've agreed that it's not a useful indication.

My issues with the current completion or used notifications are as follows:
1. There is no room for extra metadata such as checksum or flow tag.
You could put that in the descriptor payload but it's somewhat inconvenient.
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.

2. Current format assumes 1-1 corresponds between descriptors and completions.
You did offer a skipping optimization for many descriptors -> 1 completion.
But it is somewhat inefficient.
And you didn't offer a solution for 1 descriptor -> multiple completions.
Mellanox has a feature called striding RQ where you post a large buffer and
The NIC fills it with multiple back to back packets with padding.
Each packet generates its own completion.

3. There is a usage model where you have multiple produce rings
And a single completion ring.
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.

The owner bit meaning changes on each ring wrap around so the device doesn't
Need to update.

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.