Messages in this thread | | | Date | Sat, 31 Aug 2013 14:11:24 +0200 | Subject | Re: [-next] openvswitch BUILD_BUG_ON failed | From | Geert Uytterhoeven <> |
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On Fri, Aug 30, 2013 at 3:11 AM, Jesse Gross <jesse@nicira.com> wrote: > On Thu, Aug 29, 2013 at 3:10 PM, David Miller <davem@davemloft.net> wrote: >> From: Jesse Gross <jesse@nicira.com> >> Date: Thu, 29 Aug 2013 14:42:22 -0700 >> >>> On Thu, Aug 29, 2013 at 2:21 PM, Geert Uytterhoeven >>> <geert@linux-m68k.org> wrote: >>>> 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.
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.
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
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