Re: ARMv7 unaligned access

[Andrew Lunn via tcpdump-workers <tcpdump-workers () lists tcpdump org>]

> --- Begin Message ---
>
>
> From: Andrew Lunn <andrew () lunn ch>
>
> Date: Sun, 21 Aug 2022 17:56:24 +0200
>
> On Sun, Aug 21, 2022 at 11:10:53AM +0100, Denis Ovsienko via tcpdump-workers wrote:
> > Date: Sun, 21 Aug 2022 11:10:53 +0100
> > From: Denis Ovsienko <denis () ovsienko info>
> > To: tcpdump-workers () lists tcpdump org
> > Subject: ARMv7 unaligned access
> >
> > Hello list.
> >
> > Based on some prior online research, I used to think that ARMv7 is a
> > hardware that allows (by default or after simple tweaking) to terminate
> > a userspace process when it tries to perform an unaligned memory
> > access.  Which is exactly the behaviour wanted for the CI purposes.
> >
> > Not long ago I moved the linux-armv7l Buildbot worker to a true ARMv7
> > board (RPI2B v1.1).  Consistent with the documentation [1], the kernel
> > seems to have the right knob in place, which a boot-time script sets to
> > the desired position:
> >
> > $ cat /proc/cpu/alignment
> > User:         0
> > System:               0 (0x0)
> > Skipped:      0
> > Half:         0
> > Word:         0
> > DWord:                0
> > Multi:                0
> > User faults:  4 (signal)
>
> You should probably ask Russell King.
>
>     Andrew
>
>
> > However, this does not seem to have any effect, in that this short C
> > program, which does trigger a SIGBUS/SIGILL on other OSes and
> > architectures that do not like unaligned access, just completes
> > successfully on linux-armv7l:
> >
> > ---------------------------------------------------------------------
> > #include <stdint.h>
> > #include <stddef.h>
> > /*
> > #include <sys/sysmips.h>
> > */
> >
> > int main (void)
> > {
> > /*
> >       sysmips (MIPS_FIXADE, 0);
> > */
> >       uint32_t *p;
> >       union
> >       {
> >               uint32_t u32[2];
> >               uint8_t u8[8];
> >       } u;
> >       u.u32[0] = 0;
> >       u.u32[1] = 1;
> >       p = (uint32_t *)(u.u8 + 0);
> >       *p = 3;
> >       p = (uint32_t *)(u.u8 + 1);
> >       *p = 3;
> >       p = (uint32_t *)(u.u8 + 2);
> >       *p = 3;
> >       p = (uint32_t *)(u.u8 + 3);
> >       *p = 3;
> >
> >       uint8_t u8[20];
> >       for (size_t i = 0; i < 10; i++)
> >       {
> >               p = (uint32_t *)(u8 + i);
> >               *p = 4;
> >       }
> >
> >       return 0;
> > }
>
> Have you disassembled this to see what instructions it is using? The
> compiler has enough information to see you are doing unaligned
> accesses, and to use instructions which are efficient and avoid the
> unaligned access.
>
> If you do find the compiler is not optimising this, i suggest you ask
> Russell King.
>
>           Andrew
>
> --- End Message ---
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