Re: [dpdk-dev] [PATCH 00/15] Introduce Virtio vDPA driver

[Maxime Coquelin <maxime.coquelin@redhat.com>]

On 9/10/19 3:44 PM, Shahaf Shuler wrote:
> Tuesday, September 10, 2019 10:46 AM, Maxime Coquelin:
>> Subject: Re: [dpdk-dev] [PATCH 00/15] Introduce Virtio vDPA driver
>>
>> Hi Shahaf,
>>
>> On 9/9/19 1:55 PM, Shahaf Shuler wrote:
>>> Hi Maxime,
>>>
>>> Thursday, August 29, 2019 11:00 AM, Maxime Coquelin:
>>>> Subject: [dpdk-dev] [PATCH 00/15] Introduce Virtio vDPA driver
>>>>
>>>> vDPA allows to offload Virtio Datapath processing by supported NICs,
>>>> like IFCVF for example.
>>>>
>>>> The control path has to be handled by a dedicated vDPA driver, so
>>>> that it can translate Vhost-user protocol requests to proprietary
>>>> NICs registers accesses.
>>>>
>>>> This driver is the vDPA driver for Virtio devices, meaning that
>>>> Vhost-user protocol requests get translated to Virtio registers
>>>> accesses as per defined in the Virtio spec.
>>>>
>>>> Basically, it can be used within a guest with a para-virtualized
>>>> Virtio-net device, or even with a full Virtio HW offload NIC directly on
>> host.
>>>
>>> Can you elaborate more on the use cases to use such driver?
>>>
>>> 1. If the underlying HW can support full virtio device, why we need to work
>> w/ it w/ vDPA mode? Why not providing it to the VM as passthrough one?
>>> 2. why it is preferable to work w/ virtio device as the backend device to be
>> used w/ vDPA v.s. working w/ the underlying HW VF?
>>
>>
>> IMHO, I see two uses cases where it can make sense to use vDPA with a full
>> offload HW device:
>> 1. Live-migration support:  It makes it possible to switch to rings
>>    processing in SW during the migration as Virtio HH does not support
>>    dirty pages logging.
> 
> Can you elaborate why specifically using virtio_vdpa PMD enables this SW relay during migration?
> e.g. the vdpa PMD of intel that runs on top of VF do that today as well. 

I think there were a misunderstanding. When I said:
"
I see two uses cases where it can make sense to use vDPA with a full
offload HW device
"

I meant, I see two uses cases where it can make sense to use vDPA with a
full offload HW device, instead of the full offload HW device to use
Virtio PMD.

In other words, I think it is preferable to only offload the datapath,
so that it is possible to support SW live-migration.

>>
>> 2. Can be used to provide a single standard interface (the vhost-user
>>    socket) to containers in the scope of CNFs. Doing so, the container
>>    does not need to be modified, whatever the HW NIC: Virtio datapath
>>    offload only, full Virtio offload, or no offload at all. In the
>>    latter case, it would not be optimal as it implies forwarding between
>>    the Vhost PMD and the HW NIC PMD but it would work.
> 
> It is not clear to me the interface map in such system.
> From what I understand the container will have virtio-user i/f and the host will have virtio i/f. then the virtio i/f can be programmed to work w/ vDPA or not. 
> For full emulation I guess you will need to expose the netdev of the fully emulated virtio device to the container?
> 
> Am trying to map when it is beneficial to use this virtio_vdpa PMD and when it is better to use the vendor specific vDPA PMD on top of VF.

I think that with above clarification, I made it clear that the goal of
this driver is not to replace vendors vDPA drivers (their control path
maybe not even be compatible), but instead to provide a generic driver
that can be used either within a guest with a para-virtualized Virtio-
net device or with HW NIC that fully offloads Virtio (both data and
control paths).