netdev - Re: [PATCH net-next v5 2/7] net: ethtool: add tcp_data_split_mod member in kernel_ethtool

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Message-ID: <Zzep-7lOEfjXxsCs@x130>
Date: Fri, 15 Nov 2024 12:07:23 -0800
From: Saeed Mahameed <saeed@...nel.org>
To: Taehee Yoo <ap420073@...il.com>
Cc: Jakub Kicinski <kuba@...nel.org>, davem@...emloft.net,
	pabeni@...hat.com, edumazet@...gle.com, almasrymina@...gle.com,
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Subject: Re: [PATCH net-next v5 2/7] net: ethtool: add tcp_data_split_mod
 member in kernel_ethtool_ringparam

On 16 Nov 02:17, Taehee Yoo wrote:
>On Fri, Nov 15, 2024 at 1:22 PM Jakub Kicinski <kuba@...nel.org> wrote:
>>
>> On Wed, 13 Nov 2024 17:32:16 +0000 Taehee Yoo wrote:
>> > When tcp-data-split is UNKNOWN mode, drivers arbitrarily handle it.
>> > For example, bnxt_en driver automatically enables if at least one of
>> > LRO/GRO/JUMBO is enabled.
>> > If tcp-data-split is UNKNOWN and LRO is enabled, a driver returns
>> > ENABLES of tcp-data-split, not UNKNOWN.
>> > So, `ethtool -g eth0` shows tcp-data-split is enabled.
>> >
>> > The problem is in the setting situation.
>> > In the ethnl_set_rings(), it first calls get_ringparam() to get the
>> > current driver's config.
>> > At that moment, if driver's tcp-data-split config is UNKNOWN, it returns
>> > ENABLE if LRO/GRO/JUMBO is enabled.
>> > Then, it sets values from the user and driver's current config to
>> > kernel_ethtool_ringparam.
>> > Last it calls .set_ringparam().
>> > The driver, especially bnxt_en driver receives
>> > ETHTOOL_TCP_DATA_SPLIT_ENABLED.
>> > But it can't distinguish whether it is set by the user or just the
>> > current config.
>> >
>> > The new tcp_data_split_mod member indicates the tcp-data-split value is
>> > explicitly set by the user.
>> > So the driver can handle ETHTOOL_TCP_DATA_SPLIT_ENABLED properly.
>>
>> I think this can work, but it isn't exactly what I had in mind.
>>
>> I was thinking we'd simply add u8 hds_config to
>> struct ethtool_netdev_state (which is stored inside netdev).
>> And update it there if user request via ethnl_set_rings() succeeds.
>>
>> That gives the driver and the core quick and easy access to checking if
>> the user forced the setting to ENABLED or DISABLED, or didn't (UNKNOWN).
>>
>> As far as the parameter passed to ->set_ringparam() goes we could do
>> (assuming the new fields in ethtool_netdev state is called hds):
>>
>>         kernel_ringparam.tcp_data_split =
>>                 nla_get_u32_default(tb[ETHTOOL_A_RINGS_TCP_DATA_SPLIT],
>>                                     dev->ethtool->hds);
>>
>> If the driver see UNKNOWN it means user doesn't care.
>> If the driver sees ENABLED/DISABLE it must comply, doesn't matter if
>> the user requested it in current netlink call, or previous and hasn't
>> reset it, yet.
>>

This complicates things, drivers shouldn't store previous uncommitted "wanted" values.
We have wanted_features for that, and I don't think it's smart to have yet another
wanted_features mechanism, let's keep it simple, any explicit config by
ethtool should either be immediately committed or returned as error to
user and driver will only reflect the old/current value in future get requests.

HDS can conflict with many other features e.g XDP/LRO/rx_copy_break/MTU
limitations etc ...

>> Hope this makes sense...
>
>Thank you so much for the details!
>I will try to use ethtool_netdev_state instead of this approach.
>
>Thanks a lot!
>Taehee Yoo
>