Hi Joe,
On Tue, Jul 18, 2017 at 01:10:59PM -0500, Joe Hershberger wrote:
> On Wed, Jul 12, 2017 at 9:34 AM, Maxime Ripard
> <maxime.rip...@free-electrons.com> wrote:
> > The -mno-unaligned-access flag used on ARM to prevent GCC from generating
> > unaligned accesses (obviously) will only do so on packed structures.
> >
> > It seems like gcc 7.1 is a bit stricter than previous gcc versions on this,
> > and using it lead to data abort for unaligned accesses when generating
> > network traffic.
> >
> > Fix this by adding the packed attribute to the ip_udp_hdr structure in
> > order to let GCC do its job.
> >
> > Signed-off-by: Maxime Ripard <maxime.rip...@free-electrons.com>
> > ---
> > include/net.h | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/include/net.h b/include/net.h
> > index 997db9210a8f..7b815afffafa 100644
> > --- a/include/net.h
> > +++ b/include/net.h
> > @@ -390,7 +390,7 @@ struct ip_udp_hdr {
> > u16 udp_dst; /* UDP destination port */
> > u16 udp_len; /* Length of UDP packet */
> > u16 udp_xsum; /* Checksum */
> > -};
> > +} __attribute__ ((packed));
>
> Do you have an example of why this is unaligned?You can have the discussion that led to this patch in "Data abort with gcc 7.1", started a week ago. > It seems that the structure itself is naturally packed (each element > is aligned to its access size). That's true. > It seems the only time this would hit a dabort is if the head of the > buffer is not 32-bit aligned. Maybe we should address the place > where that is the case instead of forcing byte-wise accesses in > general for this structure? That's exactly what happens, the pointer to the ip_up_hdr is not aligned on 32 bits, and triggers an alignment error. However, I'm not sure how feasible it would be to align the IP packets on 32-bits, since the Ethernet header is only 14 bytes, right? We could use a bounce buffer for each packet, but that's not really optimized either. Maxime -- Maxime Ripard, Free Electrons Embedded Linux and Kernel engineering http://free-electrons.com
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