netdev - Re: [PATCH net v4 2/2] net/mlx5e: Don't overwrite pedit action when multiple pedit used

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Date:   Mon, 28 Jan 2019 17:34:10 +0200
From:   Or Gerlitz <gerlitz.or@...il.com>
To:     Tonghao Zhang <xiangxia.m.yue@...il.com>
Cc:     Saeed Mahameed <saeedm@...lanox.com>,
        Linux Netdev List <netdev@...r.kernel.org>,
        Or Gerlitz <ogerlitz@...lanox.com>
Subject: Re: [PATCH net v4 2/2] net/mlx5e: Don't overwrite pedit action when
 multiple pedit used

On Mon, Jan 28, 2019 at 2:10 PM Tonghao Zhang <xiangxia.m.yue@...il.com> wrote:
> On Mon, Jan 28, 2019 at 12:40 AM Or Gerlitz <gerlitz.or@...il.com> wrote:
> >
> > On Sun, Jan 27, 2019 at 1:06 PM <xiangxia.m.yue@...il.com> wrote:
> > > From: Tonghao Zhang <xiangxia.m.yue@...il.com>
> > >
> > > In some case, we may use multiple pedit actions to modify packets.
> > > The command shown as below: the last pedit action is effective.
> >
> > > @@ -2073,7 +2076,8 @@ static int alloc_mod_hdr_actions(struct mlx5e_priv *priv,
> > >         if (!parse_attr->mod_hdr_actions)
> > >                 return -ENOMEM;
> > >
> > > -       parse_attr->num_mod_hdr_actions = max_actions;
> > > +       parse_attr->max_mod_hdr_actions = max_actions;
> > > +       parse_attr->num_mod_hdr_actions = 0;
> >
> > why would we want to do this zeroing? what purpose does it serve?
> Because we use the num_mod_hdr_actions to store the number of actions
> we have parsed,
> and when we alloc it, we init it 0 as default.
>
> > On a probably related note, I suspect that the patch broke the caching
> > we do for modify header contexts, see mlx5e_attach_mod_hdr where we
> > look if a given set of modify header operations already has hw modify header
> > context and we use it.
> >
> > To test that, put two tc rules with different matching but same set of
> > modify header
> > (pedit) actions and see that only one modify header context is used.

> The patch does't break the cache, I think that different matching may
> share the same set of pedit actions.

I suspect it does break it.. at [1]  we have this code for the cache lookup:

num_actions  = parse_attr->num_mod_hdr_actions;
[..]
key.actions = parse_attr->mod_hdr_actions;
key.num_actions = num_actions;

hash_key = hash_mod_hdr_info(&key);

so we are doing the cached insertion and lookup with
parse_attr->num_mod_hdr_actions
which was zeroed along the way and not accounting for the full set of
pedit actions, agree?

[1] https://elixir.bootlin.com/linux/latest/source/drivers/net/ethernet/mellanox/mlx5/core/en_tc.c#L179