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From:   Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= <toke@kernel.org>
To:     Felix Fietkau <nbd@nbd.name>,
        Alexander Wetzel <alexander@wetzel-home.de>,
        Johannes Berg <johannes@sipsolutions.net>
Cc:     linux-wireless@vger.kernel.org, Pierre Asselin <pa@panix.com>
Subject: Re: [PATCH] mac80211: Use full queue selection code for control
 port tx
In-Reply-To: <65e6897d-e1ec-ffda-5f7a-4aec37621377@nbd.name>
References: <20220507083706.384513-1-alexander@wetzel-home.de>
 <87r1551t4c.fsf@toke.dk> <65e6897d-e1ec-ffda-5f7a-4aec37621377@nbd.name>
Date:   Sun, 08 May 2022 21:10:33 +0200
Message-ID: <875ymf263a.fsf@toke.dk>
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Felix Fietkau <nbd@nbd.name> writes:

> On 07.05.22 13:26, Toke H=C3=B8iland-J=C3=B8rgensen wrote:
>> Alexander Wetzel <alexander@wetzel-home.de> writes:
>>=20
>>> Calling only __ieee80211_select_queue() for control port TX exposes
>>> drivers which do not support QoS to non-zero values in
>>> skb->queue_mapping and even can assign not available queues to
>>> info->hw_queue.
>>> This can cause issues for drivers like we did e.g. see in
>>> '746285cf81dc ("rtl818x: Prevent using not initialized queues")'.
>>>
>>> This also prevents a redundant call to __ieee80211_select_queue() when
>>> using control port TX with iTXQ (pull path).
>>> And it starts to prioritize 802.11 preauthentication frames
>>> (ETH_P_PREAUTH) on all TX paths.
>>>
>>> Pierre Asselin confirmed that this patch indeed prevents crashing his
>>> system without '746285cf81dc ("rtl818x: Prevent using not initialized
>>> queues")'.
>>>
>>> Tested-by: Pierre Asselin <pa@panix.com>
>>> Signed-off-by: Alexander Wetzel <alexander@wetzel-home.de>
>>> ---
>>>
>>> Starting to prioritize ETH_P_PREAUTH was just added since I noticed that
>>> contradictory to at least my expectations control port does accept
>>> ETH_P_PREAUTH but handles these like a normal frame for the priority.
>>> That can be broken out or even drop, when needed.
>>>
>>> While looking at the code I also tripped over multiple other questions
>>> and I'll probably propose a much more invasive change how to handle
>>> the queue assignment. (End2end we seem to do some quite stupid things.)
>>>
>>> Additionally I really don't get why we call skb_get_hash() on queue
>>> assignment:
>>> I found the commit '180ac48ee62f ("mac80211: calculate skb hash early
>>> when using itxq")' but don't see why calculating the hash early is
>>> useful. Any hints here are appreciated. fq_flow_idx() seems to do that
>>> when needed and I can't find any other usage of the hash...
>>=20
>> The commit message of that commit has a hint:
>>=20
>>      This avoids flow separation issues when using software encryption.
>>=20
>> The idea being that the packet contents can change on encryption, but
>> skb->hash is preserved, so you want it to run before encryption happens
>> to keep flows in the same queue.
>>=20
>> However, AFAICT ieee80211_tx_h_encrypt() is called after frames are
>> dequeued from the TXQs, so not actually sure this is needed. Adding
>> Felix, in the hope that he can explain the reasoning behind that
>> commit :)
> Sorry, I don't remember the details on that one. One advantage that I
> can think of (which isn't mentioned in the commit msg) is that it is
> likely better for performance to calculate the hash early since there
> is a good chance that more of the skb data is still in the CPU cache.

Right, that could be, I suppose. I don't think it'll hurt, at least;
Alexander, did you have any particular reason for wanting to get rid of
it?

-Toke