netdev - RE: [net-next v2 1/8] net-zerocopy: Copy straggler unaligned data for TCP Rx. zerocopy.

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Message-ID: <1aabb09d81d24480ab2cf99e30138a3e@AcuMS.aculab.com>
Date:   Fri, 4 Dec 2020 09:02:40 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Eric Dumazet' <edumazet@...gle.com>
CC:     Stephen Hemminger <stephen@...workplumber.org>,
        Arjun Roy <arjunroy.kdev@...il.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "arjunroy@...gle.com" <arjunroy@...gle.com>,
        "soheil@...gle.com" <soheil@...gle.com>
Subject: RE: [net-next v2 1/8] net-zerocopy: Copy straggler unaligned data for
 TCP Rx. zerocopy.

From: Eric
> Sent: 03 December 2020 23:15
> 
> On Fri, Dec 4, 2020 at 12:01 AM David Laight <David.Laight@...lab.com> wrote:
> >
> > From: Stephen Hemminger
> > > Sent: 03 December 2020 00:15
> > >
> > > On Wed,  2 Dec 2020 14:09:38 -0800
> > > Arjun Roy <arjunroy.kdev@...il.com> wrote:
> > >
> > > > diff --git a/include/uapi/linux/tcp.h b/include/uapi/linux/tcp.h
> > > > index cfcb10b75483..62db78b9c1a0 100644
> > > > --- a/include/uapi/linux/tcp.h
> > > > +++ b/include/uapi/linux/tcp.h
> > > > @@ -349,5 +349,7 @@ struct tcp_zerocopy_receive {
> > > >     __u32 recv_skip_hint;   /* out: amount of bytes to skip */
> > > >     __u32 inq; /* out: amount of bytes in read queue */
> > > >     __s32 err; /* out: socket error */
> > > > +   __u64 copybuf_address;  /* in: copybuf address (small reads) */
> > > > +   __s32 copybuf_len; /* in/out: copybuf bytes avail/used or error */
> >
> > You need to swap the order of the above fields to avoid padding
> > and differing alignments for 32bit and 64bit apps.
> 
> I do not think so. Please review this patch series carefully.

Late at night.
The actual problem is 'tail padding'.

	David

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