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

[Bobby Eshleman]

On Fri, May 08, 2026 at 08:01:17AM -0700, Stanislav Fomichev wrote:
> On 05/07, Bobby Eshleman wrote:
> > From: Bobby Eshleman <bobbyeshleman@xxxxxxxx>
> > 
> > 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 <bobbyeshleman@xxxxxxxx>
> > ---
> > 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 <sdf@xxxxxxxxxxx>

Makes sense, will use the combined conditional.

Best,
Bobby