Re: [dpdk-dev] [PATCH 2/2] net/mlx5: don't map doorbell register to write combining

["Nélio Laranjeiro" <nelio.laranjeiro@6wind.com>]

On Wed, Aug 23, 2017 at 12:03:39PM +0100, Ferruh Yigit wrote:
> On 8/21/2017 8:47 AM, Sagi Grimberg wrote:
> > From: Shahaf Shuler <shahafs@mellanox.com>
> > 
> > By default, Verbs maps the doorbell register to write combining.
> > Working with write combining is useful for drivers which use blue flame
> > for the doorbell write.
> > 
> > Since mlx5 PMD uses only doorbells and write combining mapping requires
> > an extra memory barrier to flush the doorbell after its write, setting
> > the mapping to un-cached by default.
> > 
> > Such change is reduces the max and average round trip
> > latency significantly.
> > 
> > Reported-by: Alexander Solganik <solganik@gmail.com>
> > Signed-off-by: Shahaf Shuler <shahafs@mellanox.com>
> > Signed-off-by: Yongseok Koh <yskoh@mellanox.com>
> > Signed-off-by: Alexander Solganik <solganik@gmail.com>
> > Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
> > ---
> >  drivers/net/mlx5/mlx5.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/drivers/net/mlx5/mlx5.c b/drivers/net/mlx5/mlx5.c
> > index b7e5046325c0..4c2a0b9652e3 100644
> > --- a/drivers/net/mlx5/mlx5.c
> > +++ b/drivers/net/mlx5/mlx5.c
> > @@ -920,6 +920,8 @@ rte_mlx5_pmd_init(void)
> >  	 * using this PMD, which is not supported in forked processes.
> >  	 */
> >  	setenv("RDMAV_HUGEPAGES_SAFE", "1", 1);
> > +	/* Don't map UAR to WC if BlueFlame is not used.*/
> > +	setenv("MLX5_SHUT_UP_BF", "1", 1);
> 
> Although technically this is possible, I wonder how good idea it is a
> driver communicating with other processes via system environment variable?

We don't have such flexibility though verbs, those environment variable are
read by libmlx5 code and the PMD is linked though libibverbs which does not
provide a "clean" way tweak libmlx5.
It is not used to communicate with another process, it is the same process
executing the code which reads those environment variables.
Currently it is the only solution we have.

For the patch:

Acked-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>

Thanks,

-- 
Nélio Laranjeiro
6WIND