Re: [PATCH v1 1/2] vdso: Add VDSO_UNALIGNED_PACKED config option for ARMv5

[David Laight]

On Tue, 20 Oct 2026 16:56:40 +0200
Marc Kleine-Budde <mkl@xxxxxxxxxxxxxx> wrote:

> On 20.10.2026 10:24:51, Arnd Bergmann wrote:
> > > FYI: there is the kconfig Symbol HAVE_EFFICIENT_UNALIGNED_ACCESS. Can we
> > > make use of it?  
> 
> Just noticed that sashiko's review contained the same remark, but the
> bot only posts to the linux-perf-users@xxxxxxxxxxxxxxx list, where I'm
> not subscribed:
> 
> https://lore.kernel.org/all/sashiko-outbox-178182@xxxxxxxxxx/
> 
> > That would be better, but I still hope we can just get back to the
> > portable code across all architectures for the in-kernel version.
> >
> > I have one more request for you: can you check /proc/cpu/alignment
> > on the system that shows the performance regression with and without
> > the patch?  
> 
> When booted via NFS-root to the login prompt, we have about 570k System
> alignment fixups, all in the IP stack: :/
> 
> __udp4_lib_rcv+0x78/0xc6c
> icmpv6_echo_reply+0x160/0x504
> ip_send_unicast_reply+0xf0/0x3f8
> ndisc_send_skb+0x15c/0x40c
> rawv6_recvmsg+0x114/0x3b4
> tcp_rcv_established+0x3ec/0xba0
> tcp_v4_fill_cb+0x70/0xec
> tcp_validate_incoming+0x540/0x868
> 
> Comparing v7.0.12 with mainline "unaligned.h" and reverting commit
> a339671db64b ("vdso: Switch get/put_unaligned() from packed struct to
> memcpy()") brings no difference with respect to alignment problems. But
> I see the increase in I/O performance (measured with "fio" [1]) on a
> UBIFS/NAND file system:

So the nand fs is hit by the memcpy() but there are misaligned transfers
in the network stack.
 
> - v7.0.12 NFS root
>   User:           0
>   System:         573956 (tcp_measure_rcv_mss+0xe4/0x1e0)

I can't find __udp_lib_rcv(), but that looks like it could be from expanding
tcp_flag_word(tcp_hdr(skb)).
Which would rather imply that the network driver isn't correctly aligning
all receive packets (honouring NET_IP_ALIGN).

When I were a lad the kernel just paniced and userspace core dumped :-)

David