Re: [PATCH 1/2] mm: Fix struct page layout on 32-bit systems

[Arnd Bergmann <arnd@kernel.org>]

On Tue, Apr 20, 2021 at 11:14 PM Vineet Gupta
<Vineet.Gupta1@synopsys.com> wrote:
> On 4/20/21 12:07 AM, Arnd Bergmann wrote:

> >
> > which means that half the 32-bit architectures do this. This may
> > cause more problems when arc and/or microblaze want to support
> > 64-bit kernels and compat mode in the future on their latest hardware,
> > as that means duplicating the x86 specific hacks we have for compat.
> >
> > What is alignof(u64) on 64-bit arc?
>
> $ echo 'int a = __alignof__(long long);' | arc64-linux-gnu-gcc -xc -
> -Wall -S -o - | grep -A1 a: | tail -n 1 | cut -f 3
> 8

Ok, good.

> Yeah ARCv2 alignment of 4 for 64-bit data was a bit of surprise finding
> for me as well. When 64-bit load/stores were initially targeted by the
> internal Metaware compiler (llvm based) they decided to keep alignment
> to 4 still (granted hardware allowed this) and then gcc guys decided to
> follow the same ABI. I only found this by accident :-)
>
> Can you point me to some specifics on the compat issue. For better of
> worse, arc64 does''t have a compat 32-bit mode, so everything is
> 64-on-64 or 32-on-32 (ARC32 flavor of ARCv3)

In that case, there should be no problem for you.

The main issue is with system calls and ioctls that contain a misaligned
struct member like

struct s {
       u32 a;
       u64 b;
};

Passing this structure by reference from a 32-bit user space application
to a 64-bit kernel with different alignment constraints means that the
kernel has to convert the structure layout. See
compat_ioctl_preallocate() in fs/ioctl.c for one such example.

       Arnd