On Tue, Sep 3, 2013 at 11:44 PM, Jesse Gross [off-list ref] wrote:

On Sat, Aug 31, 2013 at 5:11 AM, Geert Uytterhoeven
[off-list ref] wrote:

quoted

On Fri, Aug 30, 2013 at 3:11 AM, Jesse Gross [off-list ref] wrote:

quoted

On Thu, Aug 29, 2013 at 3:10 PM, David Miller [off-list ref] wrote:

quoted

From: Jesse Gross <redacted>
Date: Thu, 29 Aug 2013 14:42:22 -0700

quoted

On Thu, Aug 29, 2013 at 2:21 PM, Geert Uytterhoeven
[off-list ref] wrote:

quoted

However, I have some doubts about other alignment "enforcements":

"__aligned(__alignof__(long))" makes the whole struct aligned to the
alignment rule for "long":
   1. This is only 2 bytes on m68k, i.e. != sizeof(long).
   2. This is 4 bytes on many 32-bit platforms, which may be less than the
      default alignment for "__be64" (cfr. some members of struct
      ovs_key_ipv4_tunnel), so this may make those 64-bit members unaligned.

Do any of those 32-bit architectures actually care about alignment of
64 bit values? On 32-bit x86, a long is 32 bits but the alignment
requirement of __be64 is also 32 bit.

All except x86-32 do, it is in fact the odd man out with respect to this
issue.

Thanks, good to know.

Andy, do you want to modify your patch to just drop the alignment
specification as Geert suggested (but definitely keep the new build
assert that you added)? It's probably better to just send the patch to
netdev (against net-next) as well since you'll likely get better
comments there and we can fix this faster if you cut out the
middleman.

Why do you want to keep the build asserts?
Is this in-memory structure also transfered as-is over the network?
If yes, you definitely want the padding.

Well they caught this bug and really don't cost anything.

quoted

Nevertheless, as the struct contains u32 and even __be64 members, the
size of the struct will always be a multiple of the alignment unit for
64-bit quantities (and thus also for long), as per the C standard.
Hence the check

    BUILD_BUG_ON(sizeof(struct sw_flow_key) % __alignof__(long));

will only catch bad compiler bugs or people adding __packed to the struct.

It's possible that we might want to pack the structure in the future.
More generally though, the contents of the struct is really
independent of the alignment requirements here because we're accessing
it as an array of bytes in long-sized chunks so implicitly depending
on the size of the members is not that great.

So you're accessing it as an array of bytes in long-sized chunks.
What are you doing with this accessed data?
Transfering over the network?
Storing on disk?
Then it must be portable across machines and architectures, right?

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds