Re: [RFC v2 bpf-next 2/7] drivers: net: turn on XDP features

["Niklas Söderlund" <niklas.soderlund@corigine.com>]

On 2023-01-17 23:15:47 +0100, Toke Høiland-Jørgensen wrote:
> Niklas Söderlund <niklas.soderlund@corigine.com> writes:
> 
> > Hi Toke,
> >
> > On 2023-01-17 22:58:57 +0100, Toke Høiland-Jørgensen wrote:
> >> Niklas Söderlund <niklas.soderlund@corigine.com> writes:
> >> 
> >> > Hi Lorenzo and Marek,
> >> >
> >> > Thanks for your work.
> >> >
> >> > On 2023-01-14 16:54:32 +0100, Lorenzo Bianconi wrote:
> >> >
> >> > [...]
> >> >
> >> >> 
> >> >> Turn 'hw-offload' feature flag on for:
> >> >>  - netronome (nfp)
> >> >>  - netdevsim.
> >> >
> >> > Is there a definition of the 'hw-offload' written down somewhere? From 
> >> > reading this series I take it is the ability to offload a BPF program?  
> >> 
> >> Yeah, basically this means "allows loading and attaching programs in
> >> XDP_MODE_HW", I suppose :)
> >> 
> >> > It would also be interesting to read documentation for the other flags 
> >> > added in this series.
> >> 
> >> Yup, we should definitely document them :)
> >> 
> >> > [...]
> >> >
> >> >> diff --git a/drivers/net/ethernet/netronome/nfp/nfp_net_common.c 
> >> >> b/drivers/net/ethernet/netronome/nfp/nfp_net_common.c
> >> >> index 18fc9971f1c8..5a8ddeaff74d 100644
> >> >> --- a/drivers/net/ethernet/netronome/nfp/nfp_net_common.c
> >> >> +++ b/drivers/net/ethernet/netronome/nfp/nfp_net_common.c
> >> >> @@ -2529,10 +2529,14 @@ static void nfp_net_netdev_init(struct nfp_net *nn)
> >> >>  	netdev->features &= ~NETIF_F_HW_VLAN_STAG_RX;
> >> >>  	nn->dp.ctrl &= ~NFP_NET_CFG_CTRL_RXQINQ;
> >> >>  
> >> >> +	nn->dp.netdev->xdp_features = NETDEV_XDP_ACT_BASIC |
> >> >> +				      NETDEV_XDP_ACT_HW_OFFLOAD;
> >> >
> >> > If my assumption about the 'hw-offload' flag above is correct I think 
> >> > NETDEV_XDP_ACT_HW_OFFLOAD should be conditioned on that the BPF firmware 
> >> > flavor is in use.
> >> >
> >> >     nn->dp.netdev->xdp_features = NETDEV_XDP_ACT_BASIC;
> >> >
> >> >     if (nn->app->type->id == NFP_APP_BPF_NIC)
> >> >         nn->dp.netdev->xdp_features |= NETDEV_XDP_ACT_HW_OFFLOAD;
> >> >
> >> >> +
> >> >>  	/* Finalise the netdev setup */
> >> >>  	switch (nn->dp.ops->version) {
> >> >>  	case NFP_NFD_VER_NFD3:
> >> >>  		netdev->netdev_ops = &nfp_nfd3_netdev_ops;
> >> >> +		nn->dp.netdev->xdp_features |= NETDEV_XDP_ACT_XSK_ZEROCOPY;
> >> >>  		break;
> >> >>  	case NFP_NFD_VER_NFDK:
> >> >>  		netdev->netdev_ops = &nfp_nfdk_netdev_ops;
> >> >
> >> > This is also a wrinkle I would like to understand. Currently NFP support 
> >> > zero-copy on NFD3, but not for offloaded BPF programs. But with the BPF 
> >> > firmware flavor running the device can still support zero-copy for 
> >> > non-offloaded programs.
> >> >
> >> > Is it a problem that the driver advertises support for both 
> >> > hardware-offload _and_ zero-copy at the same time, even if they can't be 
> >> > used together but separately?
> >> 
> >> Hmm, so the idea with this is to only expose feature flags that are
> >> supported "right now" (you'll note that some of the drivers turn the
> >> REDIRECT_TARGET flag on and off at runtime). Having features that are
> >> "supported but in a different configuration" is one of the points of
> >> user confusion we want to clear up with the explicit flags.
> >> 
> >> So I guess it depends a little bit what you mean by "can't be used
> >> together"? I believe it's possible to load two programs at the same
> >> time, one in HW mode and one in native (driver) mode, right? In this
> >> case, could the driver mode program use XSK zerocopy while the HW mode
> >> program is also loaded?
> >
> > Exactly, this is my concern. Two programs can be loaded at the same 
> > time, one in HW mode and one in native mode. The program in native mode 
> > can use zero-copy at the same time as another program runs in HW mode.
> >
> > But the program running in HW mode can never use zero-copy.
> 
> Hmm, but zero-copy is an AF_XDP feature, and AFAIK offloaded programs
> can't use AF_XDP at all? So the zero-copy "feature" is available on the
> hardware, it's just intrinsic to that feature that it doesn't work on
> offloaded programs?

That is true, so this is indeed not an issue then. Thanks for the 
clarification.

> 
> Which goes back to: yeah, we should document what the feature flags mean :)
> 
> -Toke
> 

-- 
Kind Regards,
Niklas Söderlund