Re: [dpdk-dev] [PATCH] net/mlx5: fix linkage error for glue lib

[Adrien Mazarguil <adrien.mazarguil@6wind.com>]

On Tue, Jul 24, 2018 at 02:24:12PM +0000, Yaroslav Brustinov (ybrustin) wrote:
> Hi,
> 
> Are you OK with C11?
> I saw in file dpdk/drivers/net/mlx4/mlx4_utils.c:
> 
>      * C11 code would include stdalign.h and use alignof(max_align_t) however
>      * we'll stick with C99 for the time being.
> 
> :)

Hah, this code was originally intended to become a generic EAL function
hence the C99 but got rejected. Mellanox PMDs otherwise rely on C11 features
already.

> As far as I understand, the issue is alignment of the pointer itself, not the struct.
> I'm not familiar with this command: "alignof(max_align_t)".
> Applying this on the struct:
> 
>     struct mlx5_glue {
>     ...
>     } __attribute__((__aligned__(64)));
> 
>     struct __attribute__((__aligned__(64))) mlx5_glue {
>     ...
>     };
> 
> ...still keeps the linker unhappy.

Right, this was my first (wrong) suggestion that doesn't work on types. How
about the second one instead? Here's how the diff on mlx5_glue.h should look
like:

 +#include <stdalign.h>
  #include <stddef.h>
  #include <stdint.h>

 [...]
  
 +alignas(max_align_t)
  const struct mlx5_glue *mlx5_glue;

Another comment regarding this patch, commit log should probably mention it
addresses a GCC bug that cannot be reproduced with latter versions. Keep in
mind DPDK recommends to use at least GCC version 4.9.

> -----Original Message-----
> From: Shahaf Shuler [mailto:shahafs@mellanox.com] 
> Sent: Tuesday, July 24, 2018 16:51
> To: Adrien Mazarguil <adrien.mazarguil@6wind.com>
> Cc: Yongseok Koh <yskoh@mellanox.com>; dev@dpdk.org; Yaroslav Brustinov (ybrustin) <ybrustin@cisco.com>; stable@dpdk.org; Nélio Laranjeiro <nelio.laranjeiro@6wind.com>
> Subject: RE: [PATCH] net/mlx5: fix linkage error for glue lib
> 
> Yaroslav, 
> 
> Tuesday, July 24, 2018 3:06 PM, Adrien Mazarguil:
> > Subject: Re: [PATCH] net/mlx5: fix linkage error for glue lib
> > > Odd, how could this happen assuming both files are compiled during 
> > > the same run using identical flags? Looks like a compiler issue.
> > >
> > > Anyway, may I suggest an alignment constraint on the structure type 
> > > in mlx5_glue.h instead, so that all users inherit it. E.g. using C11 syntax:
> > >
> > >  #include <stdalign.h>
> > >  #include <stddef.h>
> > >
> > >  [...]
> > >  alignas(max_align_t)
> > >  struct mlx5_glue {
> > >      [...]
> > >  };
> > 
> > My bad, this is not a correct use for alignas(), it doesn't work on types.
> > How about this instead:
> > 
> >  alignas(max_align_t)
> >  const struct mlx5_glue *mlx5_glue;
> 
> Can you confirm the above suggestion fixes your issue? 
> 
> > 
> > >

-- 
Adrien Mazarguil
6WIND