netdev - Re: [PATCH net-next v3 2/7] bnxt_en: add support for tcp-data-split ethtool command

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Message-ID: <CAMArcTVXJhJopGTHc-DqK1ydCkaQj5-VRGoJ-saGNGeTLXZHcw@mail.gmail.com>
Date: Fri, 1 Nov 2024 02:34:59 +0900
From: Taehee Yoo <ap420073@...il.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: davem@...emloft.net, pabeni@...hat.com, edumazet@...gle.com, 
	almasrymina@...gle.com, netdev@...r.kernel.org, linux-doc@...r.kernel.org, 
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	aleksander.lobakin@...el.com, dw@...idwei.uk, sridhar.samudrala@...el.com, 
	bcreeley@....com
Subject: Re: [PATCH net-next v3 2/7] bnxt_en: add support for tcp-data-split
 ethtool command

On Thu, Oct 10, 2024 at 12:28 AM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Wed, 9 Oct 2024 22:54:17 +0900 Taehee Yoo wrote:
> > > This breaks previous behavior. The HDS reporting from get was
> > > introduced to signal to user space whether the page flip based
> > > TCP zero-copy (the one added some years ago not the recent one)
> > > will be usable with this NIC.
> > >
> > > When HW-GRO is enabled HDS will be working.
> > >
> > > I think that the driver should only track if the user has set the value
> > > to ENABLED (forced HDS), or to UKNOWN (driver default). Setting the HDS
> > > to disabled is not useful, don't support it.
> >
> > Okay, I will remove the disable feature in a v4 patch.
> > Before this patch, hds_threshold was rx-copybreak value.
> > How do you think hds_threshold should still follow rx-copybreak value
> > if it is UNKNOWN mode?
>
> IIUC the rx_copybreak only applies to the header? Or does it apply
> to the entire frame?
>
> If rx_copybreak applies to the entire frame and not just the first
> buffer (headers or headers+payload if not split) - no preference.
> If rx_copybreak only applies to the headers / first buffer then
> I'd keep them separate as they operate on a different length.
>
> > I think hds_threshold need to follow new tcp-data-split-thresh value in
> > ENABLE/UNKNOWN and make rx-copybreak pure software feature.
>
> Sounds good to me, but just to be clear:
>
> If user sets the HDS enable to UNKNOWN (or doesn't set it):
>  - GET returns (current behavior, AFAIU):
>    - DISABLED (if HW-GRO is disabled and MTU is not Jumbo)
>    - ENABLED (if HW-GRO is enabled of MTU is Jumbo)
> If user sets the HDS enable to ENABLED (force HDS on):
>  - GET returns ENABLED

While I'm writing a patch I face an ambiguous problem here.
ethnl_set_ring() first calls .get_ringparam() to get current config.
Then it calls .set_ringparam() after it sets the current config + new
config to param structures.
The bnxt_set_ringparam() may receive ETHTOOL_TCP_DATA_SPLIT_ENABLED
because two cases.
1. from user
2. from bnxt_get_ringparam() because of UNKNWON.
The problem is that the bnxt_set_ringparam() can't distinguish them.
The problem scenario is here.
1. tcp-data-split is UNKNOWN mode.
2. HDS is automatically enabled because one of LRO or GRO is enabled.
3. user changes ring parameter with following command
`ethtool -G eth0 rx 1024`
4. ethnl_set_rings() calls .get_ringparam() to get current config.
5. bnxt_get_ringparam() returns ENABLE of HDS because of UNKNWON mode.
6. ethnl_set_rings() calls .set_ringparam() after setting param with
configs comes from .get_ringparam().
7. bnxt_set_ringparam() is passed ETHTOOL_TCP_DATA_SPLIT_ENABLED but
the user didn't set it explicitly.
8. bnxt_set_ringparam() eventually force enables tcp-data-split.

I couldn't find a way to distinguish them so far.
I'm not sure if this is acceptable or not.
Maybe we need to modify a scenario?

>
> hds_threshold returns: some value, but it's only actually used if GET
> returns ENABLED.
>
> > But if so, it changes the default behavior.
>
> How so? The configuration of neither of those two is exposed to
> the user. We can keep the same defaults, until user overrides them.
>
> > How do you think about it?
> >
> > >
> > > >       ering->tx_max_pending = BNXT_MAX_TX_DESC_CNT;
> > > >
> > > >       ering->rx_pending = bp->rx_ring_size;
> > > > @@ -854,9 +858,25 @@ static int bnxt_set_ringparam(struct net_device *dev,
> > > >           (ering->tx_pending < BNXT_MIN_TX_DESC_CNT))
> > > >               return -EINVAL;
> > > >
> > > > +     if (kernel_ering->tcp_data_split != ETHTOOL_TCP_DATA_SPLIT_DISABLED &&
> > > > +         BNXT_RX_PAGE_MODE(bp)) {
> > > > +             NL_SET_ERR_MSG_MOD(extack, "tcp-data-split can not be enabled with XDP");
> > > > +             return -EINVAL;
> > > > +     }
> > >
> > > Technically just if the XDP does not support multi-buffer.
> > > Any chance we could do this check in the core?
> >
> > I think we can access xdp_rxq_info with netdev_rx_queue structure.
> > However, xdp_rxq_info is not sufficient to distinguish mb is supported
> > by the driver or not. I think prog->aux->xdp_has_frags is required to
> > distinguish it correctly.
> > So, I think we need something more.
> > Do you have any idea?
>
> Take a look at dev_xdp_prog_count(), something like that but only
> counting non-mb progs?