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Message-Id: <534e8ff8-70cb-4b78-b0b4-f88645bd180a@app.fastmail.com>
Date: Tue, 16 Sep 2025 14:37:21 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Finn Thain" <fthain@...ux-m68k.org>
Cc: "Peter Zijlstra" <peterz@...radead.org>, "Will Deacon" <will@...nel.org>,
"Andrew Morton" <akpm@...ux-foundation.org>,
"Boqun Feng" <boqun.feng@...il.com>, "Jonathan Corbet" <corbet@....net>,
"Mark Rutland" <mark.rutland@....com>, linux-kernel@...r.kernel.org,
Linux-Arch <linux-arch@...r.kernel.org>,
"Geert Uytterhoeven" <geert@...ux-m68k.org>, linux-m68k@...r.kernel.org
Subject: Re: [RFC v2 3/3] atomic: Add alignment check to instrumented atomic operations
On Tue, Sep 16, 2025, at 02:16, Finn Thain wrote:
> On Mon, 15 Sep 2025, Arnd Bergmann wrote:
>
>> IIRC there are only a small number of uapi structures that need
>> __packed annotations to maintain the existing syscall ABI.
>>
>
> Packing uapi structures (and adopting -malign-int) sounds easier than the
> alternative, which might be to align certain internal kernel struct
> members, on a case-by-case basis, where doing so could be shown to improve
> performance on some architecture or other (while keeping -mno-align-int).
>
> Well, it's easy to find all the structs that belong to the uapi, but it's
> not easy to find all the internal kernel structs that describe MMIO
> registers. For -malign-int, both kinds of structs are a problem.
Right, especially since those structure definitions are more
likely to be used on older drivers, there are probably some that
do happen on m68k. On the other hand, any driver that is portable
to non-m68k targets won't have this problem.
I tried a trivial m68k defconfig build with "-Wpadded -malign-int"
and found 3021 instances of structure padding, compared to 2271
without -malign-int. That is still something one could very reasonably
go through with 'vim -q output', as almost all of them are obviously
kernel-internal structures and not problematic.
A quick manual scan only shows a number of uapi structures but very
few drivers. There are a few hardware structures that are internally
packed but in a structure that has gets an extra two bytes of harmless
padding at the end (struct CUSTOM, struct amiga_hw_present,
struct atari_hw_present, struct TT_DMA, struct ppc_regs). The only
ones I could find that actually seemed suspicious are:
arch/m68k/include/asm/atarihw.h:426:10: warning: padding struct to align 'dst_address' [-Wpadded]
arch/m68k/include/asm/openprom.h:76:13: warning: padding struct to align 'boot_dev_ctrl' [-Wpadded]
drivers/net/ethernet/i825xx/82596.c:250:1: warning: padding struct size to alignment boundary
I'm sure there are a few more, but probably not a lot more.
See https://pastebin.com/Z2bjnD0G for the full list. I've also
tried annotating the ones that show up in defconfig, see diffstat
below.
Unfortunately I found no way of annotating a struct a whole to use
the traditional padding rules: "#pragma pack(push, 2)" lowers the
alignment of all struct members to two bytes, including those
with an explicit alignment like __aligned_u64. A struct-wide
__attribute__((packed, aligned(2))) seems even worse, as it
makes all members inside of the struck tightly packed and only
aligns the struct itself.
This means all misaligned members have to be individually marked
as __packed, unless someone comes up with another type of
annotation.
> If better performance is to be had, my guess is that aligning atomic_t
> will get 80% of it (just an appeal to the Pareto principle, FWIW...)
arch/m68k selects CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS for anything
other than Dragonball, so I would not expect much performance difference
at all, unless CASL on unaligned data ends up causing alignment traps
as it does on most architectures.
Arnd
arch/m68k/atari/atakeyb.c | 2 +-
arch/m68k/include/asm/amigahw.h | 4 +--
arch/m68k/include/asm/atarihw.h | 6 ++--
arch/m68k/include/asm/mvme147hw.h | 2 +-
arch/m68k/include/asm/openprom.h | 2 +-
arch/m68k/include/uapi/asm/ptrace.h | 2 +-
arch/m68k/include/uapi/asm/sigcontext.h | 2 +-
arch/m68k/include/uapi/asm/stat.h | 4 +--
drivers/net/ethernet/i825xx/82596.c | 4 +--
include/uapi/asm-generic/termios.h | 2 +-
include/uapi/linux/acct.h | 2 +-
include/uapi/linux/ax25.h | 6 ++--
include/uapi/linux/blktrace_api.h | 2 +-
include/uapi/linux/btrfs.h | 2 +-
include/uapi/linux/cdrom.h | 36 +++++++++++-----------
include/uapi/linux/dlm.h | 2 +-
include/uapi/linux/dqblk_xfs.h | 2 +-
include/uapi/linux/ethtool.h | 14 ++++-----
include/uapi/linux/fb.h | 6 ++--
include/uapi/linux/fd.h | 6 ++--
include/uapi/linux/filter.h | 2 +-
include/uapi/linux/hdlc/ioctl.h | 6 ++--
include/uapi/linux/hiddev.h | 2 +-
include/uapi/linux/i2c-dev.h | 2 +-
include/uapi/linux/i2c.h | 2 +-
include/uapi/linux/if.h | 2 +-
include/uapi/linux/if_arcnet.h | 4 +--
include/uapi/linux/if_bonding.h | 2 +-
include/uapi/linux/if_bridge.h | 14 ++++-----
include/uapi/linux/if_link.h | 2 +-
include/uapi/linux/if_plip.h | 2 +-
include/uapi/linux/if_pppox.h | 2 +-
include/uapi/linux/if_vlan.h | 2 +-
include/uapi/linux/inet_diag.h | 2 +-
include/uapi/linux/input.h | 4 +--
include/uapi/linux/ip6_tunnel.h | 4 +--
include/uapi/linux/kd.h | 2 +-
include/uapi/linux/llc.h | 2 +-
include/uapi/linux/loop.h | 2 +-
include/uapi/linux/mctp.h | 4 +--
include/uapi/linux/mptcp.h | 2 +-
include/uapi/linux/mroute.h | 4 +--
include/uapi/linux/mroute6.h | 6 ++--
include/uapi/linux/msdos_fs.h | 2 +-
include/uapi/linux/msg.h | 6 ++--
include/uapi/linux/mtio.h | 2 +-
include/uapi/linux/netfilter/ipset/ip_set.h | 4 +--
include/uapi/linux/netfilter/nf_nat.h | 2 +-
include/uapi/linux/netfilter/x_tables.h | 8 ++---
include/uapi/linux/netfilter/xt_HMARK.h | 2 +-
include/uapi/linux/netfilter/xt_TPROXY.h | 4 +--
include/uapi/linux/netfilter/xt_connbytes.h | 2 +-
include/uapi/linux/netfilter/xt_connmark.h | 6 ++--
include/uapi/linux/netfilter/xt_conntrack.h | 4 +--
include/uapi/linux/netfilter/xt_esp.h | 2 +-
include/uapi/linux/netfilter/xt_hashlimit.h | 6 ++--
include/uapi/linux/netfilter/xt_helper.h | 2 +-
include/uapi/linux/netfilter/xt_ipcomp.h | 2 +-
include/uapi/linux/netfilter/xt_iprange.h | 2 +-
include/uapi/linux/netfilter/xt_l2tp.h | 2 +-
include/uapi/linux/netfilter/xt_mac.h | 2 +-
include/uapi/linux/netfilter/xt_mark.h | 2 +-
include/uapi/linux/netfilter/xt_owner.h | 2 +-
include/uapi/linux/netfilter/xt_realm.h | 2 +-
include/uapi/linux/netfilter/xt_recent.h | 4 +--
include/uapi/linux/netfilter/xt_set.h | 4 +--
include/uapi/linux/netfilter/xt_statistic.h | 2 +-
include/uapi/linux/netfilter/xt_tcpmss.h | 2 +-
include/uapi/linux/netfilter/xt_tcpudp.h | 2 +-
include/uapi/linux/netfilter/xt_time.h | 2 +-
include/uapi/linux/netfilter/xt_u32.h | 6 ++--
include/uapi/linux/netfilter_arp/arp_tables.h | 2 +-
include/uapi/linux/netfilter_arp/arpt_mangle.h | 2 +-
include/uapi/linux/netfilter_bridge/ebt_802_3.h | 2 +-
include/uapi/linux/netfilter_bridge/ebt_arp.h | 2 +-
include/uapi/linux/netfilter_bridge/ebt_arpreply.h | 2 +-
include/uapi/linux/netfilter_bridge/ebt_mark_m.h | 2 +-
include/uapi/linux/netfilter_bridge/ebt_nat.h | 2 +-
include/uapi/linux/netfilter_bridge/ebt_stp.h | 4 +--
include/uapi/linux/netfilter_bridge/ebt_vlan.h | 2 +-
include/uapi/linux/netfilter_ipv4/ipt_ah.h | 2 +-
include/uapi/linux/netfilter_ipv6/ip6_tables.h | 2 +-
include/uapi/linux/netfilter_ipv6/ip6t_ah.h | 2 +-
include/uapi/linux/netfilter_ipv6/ip6t_frag.h | 2 +-
include/uapi/linux/netfilter_ipv6/ip6t_opts.h | 2 +-
include/uapi/linux/netfilter_ipv6/ip6t_rt.h | 2 +-
include/uapi/linux/netfilter_ipv6/ip6t_srh.h | 2 +-
include/uapi/linux/netrom.h | 2 +-
include/uapi/linux/nfs_idmap.h | 2 +-
include/uapi/linux/nfs_mount.h | 2 +-
include/uapi/linux/pkt_cls.h | 2 +-
include/uapi/linux/rds.h | 4 +--
include/uapi/linux/rose.h | 8 ++---
include/uapi/linux/route.h | 2 +-
include/uapi/linux/rtc.h | 2 +-
include/uapi/linux/sctp.h | 18 +++++------
include/uapi/linux/sed-opal.h | 2 +-
include/uapi/linux/sem.h | 2 +-
include/uapi/linux/serial.h | 4 +--
include/uapi/linux/soundcard.h | 8 ++---
include/uapi/linux/taskstats.h | 2 +-
include/uapi/linux/virtio_net.h | 4 +--
include/uapi/linux/wireless.h | 4 +--
include/uapi/linux/xfrm.h | 26 ++++++++--------
include/uapi/mtd/mtd-abi.h | 2 +-
include/uapi/rdma/hfi/hfi1_ioctl.h | 2 +-
include/uapi/rdma/ib_user_ioctl_verbs.h | 2 +-
include/uapi/rdma/ib_user_mad.h | 2 +-
109 files changed, 205 insertions(+), 204 deletions(-)
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