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

[Shahaf Shuler <shahafs@mellanox.com>]

Tuesday, September 10, 2019 4:56 PM, Maxime Coquelin:
> Subject: Re: [dpdk-dev] [PATCH 00/15] Introduce Virtio vDPA driver
> 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,
> >>
> >>
> >> 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).

Thanks Maxim, It is clearer now. 
From what I understand this driver is to be used w/ vDPA when the underlying device is virtio. 

I can perfectly understand the para-virt ( + nested virtualization / container inside VM) use case. 

Regarding the fully emulated virtio device on the host (instead of a plain VF) - for me the benefit still not clear - if you have HW that can expose VF why not use VF + vendor specific vDPA driver.

Anyway - for the series,
Acked-by: Shahaf Shuler <shahafs@mellanox.com>