Subject: Re: [PATCH net-next v3 3/8] net: devmem: support TX over NETMEM_TX_NO_DMA devices

From: Stanislav Fomichev <[email-masked]>
Date: Fri, 8 May 2026 08:01:17 -0700

On 05/07, Bobby Eshleman wrote:
> From: Bobby Eshleman <[email-masked]>
> 
> When a netkit virtual device leases queues from a physical NIC, devmem
> TX bindings created on the netkit device must still result in the dmabuf
> being mapped for dma by the physical device. This patch accomplishes
> this by teaching the bind handler to search for the underlying
> DMA-capable device by looking it up via leased rx queues. The function
> netdev_find_netmem_tx_dev(), used for finding the underlying DMA-capable
> device, can be extended to support other non-netkit NETMEM_TX_NO_DMA
> devices in the future if needed.
> 
> Additionally, this patch extends validate_xmit_unreadable_skb() to
> support the netkit case, where the skb is validated twice: once on the
> netkit guest device and again on the physical NIC after BPF redirect or
> ip forwarding.
> 
> Signed-off-by: Bobby Eshleman <[email-masked]>
> ---
> Changes in v3:
> - Fix validate_xmit_unreadable_skb() bug for non-devmem
>   unreadable niovs (should not be dropped)
> - Major simplification of validate_xmit_unreadable_skb()
> - Fix prematurely released lock in bind-tx handler (Jakub)
> 
> Changes in v2:
> - In validate_xmit_unreadable_skb() to check netmem_tx mode before
>   inspecting frags (Jakub)
> - Lock bind_dev around netdev_queue_get_dma_dev() when bind_dev !=
>   netdev to fix lockdep (Sashiko)
> ---
>  net/core/dev.c         |  3 +++
>  net/core/devmem.c      |  6 +++--
>  net/core/devmem.h      |  9 ++++++--
>  net/core/netdev-genl.c | 63 ++++++++++++++++++++++++++++++++++++++++++++++----
>  4 files changed, 72 insertions(+), 9 deletions(-)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index fbe4c328a367..268417c9ef22 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -3999,6 +3999,9 @@ static struct sk_buff *validate_xmit_unreadable_skb(struct sk_buff *skb,
>  	if (dev->netmem_tx == NETMEM_TX_NONE)
>  		goto out_free;
>  
> +	if (dev->netmem_tx == NETMEM_TX_NO_DMA)
> +		goto out;
> +

Since this is a good case, maybe fold it into skb_frags_readable check above?

	if (likely(skb_frags_readable() || netmem_tx == NETMEM_TX_NO_DMA))

Otherwise it's a bit confusing to have:

if (xxx)
	goto out;
if (yyy)
	goto out_free;
if (zzz)
	goto out;

(or, reorder to be out/out/out_free)

Acked-by: Stanislav Fomichev <[email-masked]>