netdev - Re: [PATCH v1 net-next 04/15] net/tls: expose get_netdev_for

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Date:   Tue, 8 Dec 2020 18:06:02 -0700
From:   David Ahern <dsahern@...il.com>
To:     Boris Pismenny <borisp@...lanox.com>, kuba@...nel.org,
        davem@...emloft.net, saeedm@...dia.com, hch@....de,
        sagi@...mberg.me, axboe@...com, kbusch@...nel.org,
        viro@...iv.linux.org.uk, edumazet@...gle.com
Cc:     boris.pismenny@...il.com, linux-nvme@...ts.infradead.org,
        netdev@...r.kernel.org, benishay@...dia.com, ogerlitz@...dia.com,
        yorayz@...dia.com
Subject: Re: [PATCH v1 net-next 04/15] net/tls: expose get_netdev_for_sock

On 12/7/20 2:06 PM, Boris Pismenny wrote:
> get_netdev_for_sock is a utility that is used to obtain
> the net_device structure from a connected socket.
> 
> Later patches will use this for nvme-tcp DDP and DDP CRC offloads.
> 
> Signed-off-by: Boris Pismenny <borisp@...lanox.com>
> Reviewed-by: Sagi Grimberg <sagi@...mberg.me>
> ---
>  include/net/sock.h   | 17 +++++++++++++++++
>  net/tls/tls_device.c | 20 ++------------------
>  2 files changed, 19 insertions(+), 18 deletions(-)
> 
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 093b51719c69..a8f7393ea433 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -2711,4 +2711,21 @@ void sock_set_sndtimeo(struct sock *sk, s64 secs);
>  
>  int sock_bind_add(struct sock *sk, struct sockaddr *addr, int addr_len);
>  
> +/* Assume that the socket is already connected */
> +static inline struct net_device *get_netdev_for_sock(struct sock *sk, bool hold)
> +{
> +	struct dst_entry *dst = sk_dst_get(sk);
> +	struct net_device *netdev = NULL;
> +
> +	if (likely(dst)) {
> +		netdev = dst->dev;

I noticed you grab this once when the offload is configured. The dst
device could change - e.g., ECMP, routing changes. I'm guessing that
does not matter much for the use case - you are really wanting to
configure queues and zc buffers for a flow with the device; the netdev
is an easy gateway to get to it.

But, data center deployments tend to have redundant access points --
either multipath for L3 or bond for L2. For the latter, this offload
setup won't work - dst->dev will be the bond, the bond does not support
the offload, so user is out of luck.