linux-kernel - Re: [PATCH net-next v6 12/12] net/mlx5e: Add TX support for netmems

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Message-ID: <q7ed7rgg4uakhcc3wjxz3y6qzrvc3mhrdcpljlwfsa2a7u3sgn@6f2ronq35nee>
Date: Thu, 19 Jun 2025 07:19:52 +0000
From: Dragos Tatulea <dtatulea@...dia.com>
To: Stanislav Fomichev <stfomichev@...il.com>, 
	Mark Bloch <mbloch@...dia.com>
Cc: "David S. Miller" <davem@...emloft.net>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Eric Dumazet <edumazet@...gle.com>, Andrew Lunn <andrew+netdev@...n.ch>, 
	Simon Horman <horms@...nel.org>, saeedm@...dia.com, gal@...dia.com, leonro@...dia.com, 
	tariqt@...dia.com, Leon Romanovsky <leon@...nel.org>, 
	Jesper Dangaard Brouer <hawk@...nel.org>, Ilias Apalodimas <ilias.apalodimas@...aro.org>, 
	Richard Cochran <richardcochran@...il.com>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, John Fastabend <john.fastabend@...il.com>, 
	Stanislav Fomichev <sdf@...ichev.me>, netdev@...r.kernel.org, linux-rdma@...r.kernel.org, 
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org, Mina Almasry <almasrymina@...gle.com>
Subject: Re: [PATCH net-next v6 12/12] net/mlx5e: Add TX support for netmems

On Wed, Jun 18, 2025 at 03:16:15PM -0700, Stanislav Fomichev wrote:
> On 06/16, Mark Bloch wrote:
> > From: Dragos Tatulea <dtatulea@...dia.com>
> > 
> > Declare netmem TX support in netdev.
> > 
> > As required, use the netmem aware dma unmapping APIs
> > for unmapping netmems in tx completion path.
> > 
> > Signed-off-by: Dragos Tatulea <dtatulea@...dia.com>
> > Reviewed-by: Tariq Toukan <tariqt@...dia.com>
> > Reviewed-by: Mina Almasry <almasrymina@...gle.com>
> > Signed-off-by: Mark Bloch <mbloch@...dia.com>
> > ---
> >  drivers/net/ethernet/mellanox/mlx5/core/en/txrx.h | 3 ++-
> >  drivers/net/ethernet/mellanox/mlx5/core/en_main.c | 2 ++
> >  2 files changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en/txrx.h b/drivers/net/ethernet/mellanox/mlx5/core/en/txrx.h
> > index e837c21d3d21..6501252359b0 100644
> > --- a/drivers/net/ethernet/mellanox/mlx5/core/en/txrx.h
> > +++ b/drivers/net/ethernet/mellanox/mlx5/core/en/txrx.h
> > @@ -362,7 +362,8 @@ mlx5e_tx_dma_unmap(struct device *pdev, struct mlx5e_sq_dma *dma)
> >  		dma_unmap_single(pdev, dma->addr, dma->size, DMA_TO_DEVICE);
> >  		break;
> >  	case MLX5E_DMA_MAP_PAGE:
> > -		dma_unmap_page(pdev, dma->addr, dma->size, DMA_TO_DEVICE);
> > +		netmem_dma_unmap_page_attrs(pdev, dma->addr, dma->size,
> > +					    DMA_TO_DEVICE, 0);
> 
> For this to work, the dma->addr needs to be 0, so the callers of the
> dma_map() need to be adjusted as well, or am I missing something?
> There is netmem_dma_unmap_addr_set to handle that, but I don't see
> anybody calling it. Do we need to add the following (untested)?
>
Hmmmm... yes. I figured that skb_frag_dma_map() would do the work
but I was wrong, it is not enough.

> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_tx.c b/drivers/net/ethernet/mellanox/mlx5/core/en_tx.c
> index 55a8629f0792..fb6465210aed 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/en_tx.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en_tx.c
> @@ -210,7 +210,9 @@ mlx5e_txwqe_build_dsegs(struct mlx5e_txqsq *sq, struct sk_buff *skb,
>  		if (unlikely(dma_mapping_error(sq->pdev, dma_addr)))
>  			goto dma_unmap_wqe_err;
>  
> -		dseg->addr       = cpu_to_be64(dma_addr);
> +		dseg->addr = 0;
> +		if (!netmem_is_net_iov(skb_frag_netmem(frag)))
> +			dseg->addr = cpu_to_be64(dma_addr);
AFAIU we still want to pass the computed dma_address to the data segment
to the HW. We only need to make sure in mlx5e_dma_push() to set dma_addr
to 0, to avoid calling netmem_dma_unmap_page_attrs() with dma->addr 0.
Like in the snippet below. Do you agree?

We will send a fix patch once the above question is answered. Also, is
there a way to test this with more confidence? The ncdevmem tx test
passed just fine.

Thanks,
Dragos

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_tx.c b/drivers/net/ethernet/mellanox/mlx5/core/en_tx.c
index 55a8629f0792..ecee2e4f678b 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/en_tx.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/en_tx.c
@@ -214,6 +214,9 @@ mlx5e_txwqe_build_dsegs(struct mlx5e_txqsq *sq, struct sk_buff *skb,
                dseg->lkey       = sq->mkey_be;
                dseg->byte_count = cpu_to_be32(fsz);
 
+               if (!netmem_is_net_iov(skb_frag_netmem(frag)))
+                       dma_addr = 0;
+
                mlx5e_dma_push(sq, dma_addr, fsz, MLX5E_DMA_MAP_PAGE);
                num_dma++;
                dseg++;