netdev - Re: [PATCH net 1/1] openvswitch: Fix setting ipv6 fields causing hw csum failure

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Message-ID: <20220210102151.6df356ba@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Thu, 10 Feb 2022 10:21:51 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Paul Blakey <paulb@...dia.com>
Cc:     <dev@...nvswitch.org>, <netdev@...r.kernel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Pravin B Shelar <pshelar@....org>, <davem@...emloft.net>,
        Eelco Chaudron <echaudro@...hat.com>,
        Oz Shlomo <ozsh@...dia.com>, "Vlad Buslov" <vladbu@...dia.com>,
        Roi Dayan <roid@...dia.com>,
        Ariel Levkovich <lariel@...dia.com>
Subject: Re: [PATCH net 1/1] openvswitch: Fix setting ipv6 fields causing hw
 csum failure

On Thu, 10 Feb 2022 10:53:24 +0200 Paul Blakey wrote:
> > The calls seem a little heavy for single byte replacements.
> > Can you instead add a helper based on csum_replace4() maybe?
> > 
> > BTW doesn't pedit have the same problem?
> 
> I don't think they are heavier then csum_replace4,

csum_replace4 is a handful of instructions all of which will be inlined.
csum_partial() is a function call and handles variable lengths.

> but they are more bulletproof in my opinion, since they handle both
> the COMPLETE and PARTIAL csum cases (in __skb_postpull_rcsum())

Yes, that's why I said "add a helper based on", a skb helper which
checks the csum type of the packet but instead of calling csum_partial
for no reason does the adjustment directly.

> and resemble what editing of the packet should have done - pull the
> header, edit, and then push it back.

That's not what this code is doing, so the argument does not stand IMO.