netdev - RE: [PATCH net-next 2/8] i40e: change Rx buffer size for legacy-rx to support XDP multi-buffer

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Date:   Wed, 8 Mar 2023 12:22:13 +0000
From:   "Sarkar, Tirthendu" <tirthendu.sarkar@...el.com>
To:     Jakub Kicinski <kuba@...nel.org>,
        "Nguyen, Anthony L" <anthony.l.nguyen@...el.com>
CC:     "davem@...emloft.net" <davem@...emloft.net>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "Fijalkowski, Maciej" <maciej.fijalkowski@...el.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "hawk@...nel.org" <hawk@...nel.org>,
        "john.fastabend@...il.com" <john.fastabend@...il.com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "Rout, ChandanX" <chandanx.rout@...el.com>
Subject: RE: [PATCH net-next 2/8] i40e: change Rx buffer size for legacy-rx to
 support XDP multi-buffer

> -----Original Message-----
> From: Jakub Kicinski <kuba@...nel.org>
> Subject: Re: [PATCH net-next 2/8] i40e: change Rx buffer size for legacy-rx to
> support XDP multi-buffer
> 
> On Mon,  6 Mar 2023 13:08:16 -0800 Tony Nguyen wrote:
> > In the legacy-rx mode, driver can only configure up to 2k sized Rx buffers
> > and with the current configuration of 2k sized Rx buffers there is no way
> > to do tailroom reservation for skb_shared_info. Hence size of Rx buffers
> > is now lowered to 1664 (2k - sizeof(skb_shared_info)). Also, driver can
> 
> skb_shared_info is not fixed size, the number of fragments can
> be changed in the future. What will happen to the driver and
> this assumption, then?
> 

This is for the non-default path in legacy mode. If for some reason number of 
fragments increase in future, we may have to think of other options like using
page pools.

> > only chain up to 5 Rx buffers and this means max MTU supported for
> > legacy-rx is now 8320.