RE: [PATCH net-next 06/10] net/mlx4_core: Fix struct mlx4_vhcr_cmd to make implicit padding explicit

[David Laight <David.Laight@ACULAB.COM>]

From: Jack Morgenstein
> On Tue, 27 Jan 2015 09:43:27 +0000
> David Laight <David.Laight@ACULAB.COM> wrote:
> 
> > From: Amir Vadai
> > > From: Jack Morgenstein <jackm@dev.mellanox.co.il>
> > >
> > > Struct mlx4_vhcr was implicitly padded by the gcc compiler.
> > > This commit makes that padding explicit, to prevent issues with
> > > changing compilers. Note that we added the padding dword (rather
> > > than simply packing the structure) in order to maintain
> > > compatibility with previous kernels.
> >
> > It isn't a 'compiler' option, but depends on the architecture.
> >
> > > Reported-by: Alexander Schmidt <alexs@linux.vnet.ibm.com>
> > > Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
> > > Signed-off-by: Amir Vadai <amirv@mellanox.com>
> > > ---
> > >  drivers/net/ethernet/mellanox/mlx4/mlx4.h | 3 ++-
> > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/net/ethernet/mellanox/mlx4/mlx4.h
> > > b/drivers/net/ethernet/mellanox/mlx4/mlx4.h index 096a81c..595e18a
> > > 100644 --- a/drivers/net/ethernet/mellanox/mlx4/mlx4.h
> > > +++ b/drivers/net/ethernet/mellanox/mlx4/mlx4.h
> > > @@ -196,13 +196,14 @@ struct mlx4_vhcr {
> > >  struct mlx4_vhcr_cmd {
> > >  	__be64 in_param;
> > >  	__be32 in_modifier;
> > > +	u32 reserved1;
> >
> > Adding a pad here changes the alignment on most 32bit architectures
> > (eg i386) where 64bit integers are only aligned to 4 byte boundaries.
> > So you are not 'maintaining compatibility with previous kernels'.
> 
> You are correct. However, this structure is used ONLY for communication
> between a Host and a Guest on that host.  In the current code (before
> this fix), it was not possible to run a 32-bit Guest over a 64-bit Host,
> because of the implicit padding that occurred on the 64-bit Host
> and did not occur on the 32-bit Guest.
> 
> With this fix, a 32-bit Guest (with the fix) will be able to run over a
> 64-bit Host (with or without the fix).

Haven't you broken communication between an old and new
32bit Guests and 32bit hosts (assuming they exist)?

	David