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

[Adrien Mazarguil]

On Wed, Jul 25, 2018 at 07:38:39AM +0000, Yaroslav Brustinov (ybrustin) wrote:
> +Hanoch
> 
> I've added -std=c11 flag to our compilation to check.
> 
> Without alignas(max_align_t):
> /usr/bin/ld: Warning: alignment 8 of symbol `mlx5_glue' in src/dpdk/drivers/net/mlx5/mlx5_glue.c.11.o is smaller than 32 in src/dpdk/drivers/net/mlx5/mlx5_rxq.c.11.o
> 
> With alignas(max_align_t):
> /usr/bin/ld: Warning: alignment 16 of symbol `mlx5_glue' in src/dpdk/drivers/net/mlx5/mlx5_glue.c.11.o is smaller than 32 in src/dpdk/drivers/net/mlx5/mlx5_rxq.c.11.o
> 
> Using alignas(64) does not produce linker warning.

OK, let's forget max_align_t. Even better, how about
alignas(RTE_CACHE_LINE_SIZE) just in case the same GCC version complains
about the lack of a 128 byte alignment on architectures like IBM POWER8.

(remember to #include <rte_config.h> for RTE_CACHE_LINE_SIZE)

> -----Original Message-----
> From: Adrien Mazarguil [mailto:adrien.mazarguil at 6wind.com] 
> Sent: Tuesday, July 24, 2018 19:04
> To: Yaroslav Brustinov (ybrustin) <ybrustin at cisco.com>
> Cc: Shahaf Shuler <shahafs at mellanox.com>; Yongseok Koh <yskoh at mellanox.com>; dev at dpdk.org; stable at dpdk.org; Nélio Laranjeiro <nelio.laranjeiro at 6wind.com>
> Subject: Re: [PATCH] net/mlx5: fix linkage error for glue lib
> 
> 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 at mellanox.com]
> > Sent: Tuesday, July 24, 2018 16:51
> > To: Adrien Mazarguil <adrien.mazarguil at 6wind.com>
> > Cc: Yongseok Koh <yskoh at mellanox.com>; dev at dpdk.org; Yaroslav 
> > Brustinov (ybrustin) <ybrustin at cisco.com>; stable at dpdk.org; Nélio 
> > Laranjeiro <nelio.laranjeiro at 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

-- 
Adrien Mazarguil
6WIND