linux-kernel - Re: [PATCH net-next] stmmac: align RX buffers

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Message-ID: <CAFnufp0N2MzaTjF95tx9Q1D33z9f9AAK6UHbhU9rhG1ue_r1ug@mail.gmail.com>
Date:   Fri, 20 Aug 2021 19:14:30 +0200
From:   Matteo Croce <mcroce@...ux.microsoft.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Thierry Reding <thierry.reding@...il.com>,
        netdev@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Drew Fustini <drew@...gleboard.org>,
        Emil Renner Berthing <kernel@...il.dk>,
        Jon Hunter <jonathanh@...dia.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH net-next] stmmac: align RX buffers

On Fri, Aug 20, 2021 at 7:09 PM Marc Zyngier <maz@...nel.org> wrote:
>
> On Fri, 20 Aug 2021 17:38:14 +0100,
> Matteo Croce <mcroce@...ux.microsoft.com> wrote:
> >
> > On Fri, Aug 20, 2021 at 6:26 PM Marc Zyngier <maz@...nel.org> wrote:
> > >
> > > On Fri, 20 Aug 2021 11:37:03 +0100,
> > > Matteo Croce <mcroce@...ux.microsoft.com> wrote:
> > > >
> > > > On Thu, Aug 19, 2021 at 6:29 PM Marc Zyngier <maz@...nel.org> wrote:
> > >
> > > [...]
> > >
> > > > > diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac.h b/drivers/net/ethernet/stmicro/stmmac/stmmac.h
> > > > > index fcdb1d20389b..244aa6579ef4 100644
> > > > > --- a/drivers/net/ethernet/stmicro/stmmac/stmmac.h
> > > > > +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac.h
> > > > > @@ -341,7 +341,7 @@ static inline unsigned int stmmac_rx_offset(struct stmmac_priv *priv)
> > > > >         if (stmmac_xdp_is_enabled(priv))
> > > > >                 return XDP_PACKET_HEADROOM + NET_IP_ALIGN;
> > > > >
> > > > > -       return NET_SKB_PAD + NET_IP_ALIGN;
> > > > > +       return 8 + NET_IP_ALIGN;
> > > > >  }
> > > > >
> > > > >  void stmmac_disable_rx_queue(struct stmmac_priv *priv, u32 queue);
> > > > >
> > > > > I don't see the system corrupting packets anymore. Is that exactly
> > > > > what you had in mind? This really seems to point to a basic buffer
> > > > > overflow.
> > >
> > > [...]
> > >
> > > > Sorry, I meant something like:
> > > >
> > > > -       return NET_SKB_PAD + NET_IP_ALIGN;
> > > > +       return 8;
> > > >
> > > > I had some hardware which DMA fails if the receive buffer was not word
> > > > aligned, but this seems not the case, as 8 + NET_IP_ALIGN = 10, and
> > > > it's not aligned too.
> > >
> > > No error in that case either, as expected. Given that NET_SKB_PAD is
> > > likely to expand to 64, it is likely a DMA buffer overflow which
> > > probably only triggers for large-ish packets.
> > >
> > > Now, we're almost at -rc7, and we don't have a solution in sight.
> > >
> > > Can we please revert this until we have an understanding of what is
> > > happening? I'll hopefully have more cycles to work on the issue once
> > > 5.14 is out, and hopefully the maintainers of this driver can chime in
> > > (they have been pretty quiet so far).
> > >
> > > Thanks,
> > >
> > >         M.
> > >
> > > --
> > > Without deviation from the norm, progress is not possible.
> >
> > Last try, what about adding only NET_IP_ALIGN and leaving NET_SKB_PAD?
> >
> > -       return NET_SKB_PAD + NET_IP_ALIGN;
> > +       return NET_IP_ALIGN;
> >
> > I think that alloc_skb adds another NET_SKB_PAD anyway.
>
> I don't see any packet corruption with this. However, this doesn't
> prove that this is correct either. What was the rational for adding
> NET_SKB_PAD the first place?
>

I think it's wrong. The original offset was 0, and to align it to the
boundary we need to add just NET_IP_ALIGN, which is two.
NET_SKB_PAD is a much bigger value, (I think 64), which is used to
reserve space to prepend an header, e.g. with tunnels.

-- 
per aspera ad upstream