netdev - Re: [PATCH 01/12 v2 RFC] net: group skb_shinfo zerocopy related bits together.

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Date:   Tue, 22 Dec 2020 17:56:09 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Jonathan Lemon <jonathan.lemon@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH 01/12 v2 RFC] net: group skb_shinfo zerocopy related bits together.

On Tue, Dec 22, 2020 at 5:40 PM Jonathan Lemon <jonathan.lemon@...il.com> wrote:
>
> On Tue, Dec 22, 2020 at 05:26:15PM -0500, Willem de Bruijn wrote:
> > On Tue, Dec 22, 2020 at 12:22 PM Jonathan Lemon
> > <jonathan.lemon@...il.com> wrote:
> > >
> > > On Tue, Dec 22, 2020 at 09:43:15AM -0500, Willem de Bruijn wrote:
> > > > On Mon, Dec 21, 2020 at 7:09 PM Jonathan Lemon <jonathan.lemon@...il.com> wrote:
> > > > >
> > > > > From: Jonathan Lemon <bsd@...com>
> > > > >
> > > > > In preparation for expanded zerocopy (TX and RX), move
> > > > > the ZC related bits out of tx_flags into their own flag
> > > > > word.
> > > > >
> > > > > Signed-off-by: Jonathan Lemon <jonathan.lemon@...il.com>
> > > >
> > > > I think it's better to expand tx_flags to a u16 and add the two new
> > > > flags that you need.
> > >
> > > Okay, but in that case, tx_flags is now wrong, since some of these flags
> > > will also be checked on the rx path.
> > >
> > >
> > > > That allows the additional 7 bits to be used for arbitrary flags, not
> > > > stranding 8 bits exactly only for zerocopy features.
> > > >
> > > > Moving around a few u8's in the same cacheline won't be a problem.
> > >
> > > How about rearranging them to form 16 bits, and just calling it 'flags'?
> >
> > I thought that would be a good idea, but tx_flags is used in a lot
> > more places than I expected. That will be a lot of renaming. Let's
> > just either keep the name or add a new field.
>
> Hmm, which?  I went with "add new field" = zc_flags.  I can rename this
> to something else if that's too specific.  Suggestions?

Just flags, for now? High order bit is to not move and rename all the
existing flags for the sake of it. Name itself is less important.