| Message ID | CAMuHMdVG9FmZNayrf7HMz4kC4X5QELeXUFjjzpAM80ND_QOm8A@mail.gmail.com |
|---|---|
| State | RFC, archived |
| Delegated to: | David Miller |
| Headers | show |
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. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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 -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Sat, Aug 31, 2013 at 5:11 AM, Geert Uytterhoeven <geert@linux-m68k.org> wrote: > 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. Well they caught this bug and really don't cost anything. > 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. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Tue, Sep 3, 2013 at 11:44 PM, Jesse Gross <jesse@nicira.com> wrote: > On Sat, Aug 31, 2013 at 5:11 AM, Geert Uytterhoeven > <geert@linux-m68k.org> wrote: >> 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. > > Well they caught this bug and really don't cost anything. > >> 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 -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Tue, Sep 3, 2013 at 11:55 PM, Geert Uytterhoeven <geert@linux-m68k.org> wrote: > On Tue, Sep 3, 2013 at 11:44 PM, Jesse Gross <jesse@nicira.com> wrote: >> On Sat, Aug 31, 2013 at 5:11 AM, Geert Uytterhoeven >> <geert@linux-m68k.org> wrote: >>> 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. >> >> Well they caught this bug and really don't cost anything. >> >>> 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? It's just an in-memory hash table lookup. No one else ever sees it. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
diff --git a/net/openvswitch/flow.h b/net/openvswitch/flow.h index b65f885..15f08d9 100644 --- a/net/openvswitch/flow.h +++ b/net/openvswitch/flow.h @@ -78,6 +78,7 @@ struct sw_flow_key { u32 priority; /* Packet QoS priority. */ u32 skb_mark; /* SKB mark. */ u16 in_port; /* Input switch port (or DP_MAX_PORTS). + u16 pad; } phy; struct { u8 src[ETH_ALEN]; /* Ethernet source address. */
On m68k, where the alignment of 32-bit words is 2 bytes: net/openvswitch/flow.c:1984:2: error: call to '__compiletime_assert_1984' declared with attribute error: BUILD_BUG_ON failed: sizeof(struct sw_flow_key) % sizeof(long) (http://kisskb.ellerman.id.au/kisskb/buildresult/9422860/) This was introduced by commit 5828cd9a68873df1340b420371c02c47647878fb Author: Andy Zhou <azhou@nicira.com> Date: Tue Aug 27 13:02:21 2013 -0700 openvswitch: optimize flow compare and mask functions Make sure the sw_flow_key structure and valid mask boundaries are always machine word aligned. Optimize the flow compare and mask operations using machine word size operations. This patch improves throughput on average by 15% when CPU is the bottleneck of forwarding packets. This patch is inspired by ideas and code from a patch submitted by Peter Klausler titled "replace memcmp() with specialized comparator". However, The original patch only optimizes for architectures support unaligned machine word access. This patch optimizes for all architectures. A quick fix to satisfy the build check is to make the padding explicit (gmail-whitespace-damaged diff): 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. I guess you want (at least) 4 byte alignment on 32-bit, and prefer 8 byte alignment on 64-bit? Not specifying any alignment constraint will give you most of that (except on 64-bit platforms where 64-bit words must be only 4-byte aligned). There's another build check "BUILD_BUG_ON(sizeof(long) % sizeof(u32))". Isn't this always true on Linux, as "long" is never smaller than 4 bytes? 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 -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html