Regarding Mellanox bifurcated driver on Azure

[Stephen Hemminger]

On Wed, 30 Apr 2025 22:00:29 +0530
Prashant Upadhyaya <praupadhyaya at gmail.com> wrote:

> On Wed, 30 Apr 2025 at 19:58, Stephen Hemminger <stephen at networkplumber.org>
> wrote:
> 
> > On Wed, 30 Apr 2025 13:00:53 +0530
> > Prashant Upadhyaya <praupadhyaya at gmail.com> wrote:
> >  
> > > > With DPDK on Azure, an application should never use the VF directly.
> > > > It needs to use either netvsc PMD which handles both the vmbus (slow  
> > path)  
> > > > and VF (fast path) combined. Or use the older vdev_netvsc/failsafe/tap
> > > > combination.
> > > > The latter uses a virtual device to make a failsafe PMD which then does
> > > > a combination of TAP (via kernel slow path) and MLX5 VF.  The failsafe  
> > PMD  
> > > > is what is exposed for application usage.
> > > >
> > > > The limitations are not explicitly mentioned in the documentation but:
> > > >   - don't use VF directly in application
> > > >   - there is no support for bifurcation where some packets go to kernel
> > > >     and some to DPDK
> > > >   - there is only very limited support for rte_flow; that is with  
> > failsafe  
> > > > PMD
> > > >     (not netvsc PMD) and the limitations are that the emulation of  
> > rte_flow  
> > > >     in the TAP device only supports a few things.
> > > >  
> > >
> > > Thanks Stephen, the above information was very instructive.
> > > If I do use the Netvsc PMD with the latest DPDK, will my DPDK app get the
> > > non IP packets like ARP, please confirm.
> > > I quickly tried the Netvsc PMD but don't seem to be getting the ARP  
> > packets  
> > > in still.
> > > When you mention "The failsafe PMD is what is exposed for application
> > > usage", what is the meaning of this, are the apps expected to use  
> > failsafe  
> > > PMD, please suggest.
> > >
> > > Regards
> > > -Prashant  
> >
> > ARP handled differently in virtual network environments. The ARP packets
> > sent
> > get consumed and replied to by the network infrastructure (in all virtual
> > networks
> > not just Azure). Non-IP packets always show up on the synthetic VMBus
> > device.
> >
> > Current docs are here:
> >
> > https://learn.microsoft.com/en-us/azure/virtual-network/setup-dpdk?tabs=redhat
> >
> > See vdev_netvsc for picture.
> > https://doc.dpdk.org/guides/nics/vdev_netvsc.html
> >
> >
> > Thanks again Stephen. I finally was able to run the netvsc pmd and it is  
> detecting the ports.
> However, for every accelerated networking interface of Azure, it detects
> 'two' ports. This is presumably for controlling both the slow and fast path
> ?
> This poses an issue for my app as it wanted to see only 'one' interface in
> its control as a lot of business logic is kind of tied to it.
> So a question -- am I observing correctly that DPDK, in case of netvsc,
> will enumerate two ports for each accelerated networking interface ?
> 
> Regards
> -Prashant

The hidden VF interfaces are "owned" in DPDK.
If you use the standard API's in ethdev it will skip the owned interfaces.

/**
 * Macro to iterate over all enabled and ownerless ethdev ports.
 */
#define RTE_ETH_FOREACH_DEV(p) \
	RTE_ETH_FOREACH_DEV_OWNED_BY(p, RTE_ETH_DEV_NO_OWNER)