On 10/05/2012 08:51 AM, Mikael Pettersson wrote:
> Rob Herring writes:
> > On 10/05/2012 03:24 AM, Russell King - ARM Linux wrote:
> > > On Fri, Oct 05, 2012 at 09:20:56AM +0100, Mans Rullgard wrote:
> > >> On 5 October 2012 08:12, Russell King - ARM Linux
> > >> <li...@arm.linux.org.uk> wrote:
> > >>> On Fri, Oct 05, 2012 at 03:25:16AM +0100, Mans Rullgard wrote:
> > >>>> On 5 October 2012 02:56, Rob Herring <robherri...@gmail.com> wrote:
> > >>>>> This struct is the IP header, so a struct ptr is just set to the
> > >>>>> beginning of the received data. Since ethernet headers are 14 bytes,
> > >>>>> often the IP header is not aligned unless the NIC can place the
> frame at
> > >>>>> a 2 byte offset (which is something I need to investigate). So this
> > >>>>> function cannot make any assumptions about the alignment. Does the
> ABI
> > >>>>> define structs have some minimum alignment? Does the struct need to
> be
> > >>>>> declared as packed or something?
> > >>>>
> > >>>> The ABI defines the alignment of structs as the maximum alignment of
> its
> > >>>> members. Since this struct contains 32-bit members, the alignment
> for the
> > >>>> whole struct becomes 32 bits as well. Declaring it as packed tells
> gcc it
> > >>>> might be unaligned (in addition to removing any holes within).
> > >>>
> > >>> This has come up before in the past.
> > >>>
> > >>> The Linux network folk will _not_ allow - in any shape or form - for
> > >>> this struct to be marked packed (it's the struct which needs to be
> > >>> marked packed) because by doing so, it causes GCC to issue byte loads/
> > >>> stores on architectures where there isn't a problem, and that decreases
> > >>> the performance of the Linux IP stack unnecessarily.
> > >>
> > >> Which architectures? I have never seen anything like that.
> > >
> > > Does it matter? I'm just relaying the argument against adding __packed
> > > which was used before we were forced (by the networking folk) to
> implement
> > > the alignment fault handler.
> >
> > It doesn't really matter what will be accepted or not as adding __packed
> > to struct iphdr doesn't fix the problem anyway. gcc still emits a ldm.
> > The only way I've found to eliminate the alignment fault is adding a
> > barrier between the 2 loads. That seems like a compiler issue to me if
> > there is not a better fix.
>
> If you suspect a GCC bug, please prepare a standalone user-space test case
> and submit it to GCC's bugzilla (I can do the latter if you absolutely do not
> want to). It wouldn't be the first alignment-related GCC bug...
>
Here's a testcase. Compiled on ubuntu precise with
"arm-linux-gnueabihf-gcc -O2 -marm -march=armv7-a test.c".
typedef unsigned short u16;
typedef unsigned short __sum16;
typedef unsigned int __u32;
typedef unsigned char __u8;
typedef __u32 __be32;
typedef u16 __be16;
struct iphdr {
__u8 ihl:4,
version:4;
__u8 tos;
__be16 tot_len;
__be16 id;
__be16 frag_off;
__u8 ttl;
__u8 protocol;
__sum16 check;
__be32 saddr;
__be32 daddr;
/*The options start here. */
};
#define ntohl(x) __swab32((__u32)(__be32)(x))
#define IP_DF 0x4000 /* Flag: "Don't Fragment" */
static inline __attribute__((const)) __u32 __swab32(__u32 x)
{
__asm__ ("rev %0, %1" : "=r" (x) : "r" (x));
return x;
}
int main(void * buffer, unsigned int *p_id)
{
unsigned int id;
int flush = 1;
const struct iphdr *iph = buffer;
__u32 len = *p_id;
id = ntohl(*(__be32 *)&iph->id);
flush = (u16)((ntohl(*(__be32 *)iph) ^ len) | (id ^ IP_DF));
id >>= 16;
*p_id = id;
return flush;
}
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