netdev - RE: [PATCH V1 net-next 0/5] Add devlink support to ena

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Date:   Wed, 11 Jan 2023 21:21:16 +0000
From:   "Arinzon, David" <darinzon@...zon.com>
To:     Jakub Kicinski <kuba@...nel.org>
CC:     David Miller <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "Machulsky, Zorik" <zorik@...zon.com>,
        "Matushevsky, Alexander" <matua@...zon.com>,
        "Bshara, Saeed" <saeedb@...zon.com>,
        "Bshara, Nafea" <nafea@...zon.com>,
        "Saidi, Ali" <alisaidi@...zon.com>,
        "Kiyanovski, Arthur" <akiyano@...zon.com>,
        "Dagan, Noam" <ndagan@...zon.com>,
        "Agroskin, Shay" <shayagr@...zon.com>,
        "Itzko, Shahar" <itzko@...zon.com>,
        "Abboud, Osama" <osamaabb@...zon.com>,
        "Schmeilin, Evgeny" <evgenys@...zon.com>
Subject: RE: [PATCH V1 net-next 0/5] Add devlink support to ena



> -----Original Message-----
> From: Jakub Kicinski <kuba@...nel.org>
> Sent: Wednesday, January 11, 2023 10:00 PM
> To: Arinzon, David <darinzon@...zon.com>
> Cc: David Miller <davem@...emloft.net>; netdev@...r.kernel.org;
> Machulsky, Zorik <zorik@...zon.com>; Matushevsky, Alexander
> <matua@...zon.com>; Bshara, Saeed <saeedb@...zon.com>; Bshara,
> Nafea <nafea@...zon.com>; Saidi, Ali <alisaidi@...zon.com>;
> Kiyanovski, Arthur <akiyano@...zon.com>; Dagan, Noam
> <ndagan@...zon.com>; Agroskin, Shay <shayagr@...zon.com>; Itzko,
> Shahar <itzko@...zon.com>; Abboud, Osama <osamaabb@...zon.com>
> Subject: RE: [EXTERNAL][PATCH V1 net-next 0/5] Add devlink support to
> ena
> 
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> 
> On Wed, 11 Jan 2023 19:31:39 +0000 Arinzon, David wrote:
> > If the packet network headers are not within the size of the LLQ
> > entry, then the packet will be dropped. So I'll say that c) describes
> > the impact the best given that certain types of traffic will not succeed or
> have disruptions due to dropped TX packets.
> 
> I see. Sounds like it could be a fit for
> DEVLINK_ATTR_ESWITCH_INLINE_MODE ? But that one configures the
> depth of the headers copied inline, rather than bytes. We could add a value
> for "tunneled" and have that imply 256B LLQ in case of ena.
> 
> The other option is to introduce the concept of "max length of inline data"
> to ethtool, and add a new netlink attribute to ethtool -g.
> 
> Either way - the length of "inline|push" data is not a ena-specific concept,
> so using private knobs (devlink params or ethtool flags) is not appropriate
> upstream. We should add a bona fide uAPI for it.

I've looked into the original commit of DEVLINK_ATTR_ESWITCH_INLINE_MODE
and it seems to be more related to encapsulation and adding a tunneling concept
here would be misleading, as it doesn't cover all the use-cases affected here.
It is true that in case of tunneling and if there's a need to access the encapsulated
headers, the length of the headers might be larger than the default value in LLQ,
but this may be also the case for non-tunneled traffic, for example,
IPv6 with TCP SACK option.

I'll note again that this is not a configurable value, meaning, the only option is
to have 128B (standard) or 256B (large) LLQ, there's no option to set other values,
but only choose between the modes, therefore, I don't see how having such an
option through ethtool, as you suggested (setting the max length) can be
beneficial in our use-case (might be good overall, as you noted, it's a more
generic concept). Will put more thought into it.