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From:   Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= <toke@redhat.com>
To:     Felix Fietkau <nbd@nbd.name>, linux-wireless@vger.kernel.org
Cc:     johannes@sipsolutions.net
Subject: Re: [PATCH 2/7] mac80211: force calculation of software hash for tx
 fair queueing
In-Reply-To: <add3d1fa-c2ad-5aaf-83c7-31e919129bbf@nbd.name>
References: <20201216204316.44498-1-nbd@nbd.name>
 <20201216204316.44498-2-nbd@nbd.name> <87czz8bqbj.fsf@toke.dk>
 <add3d1fa-c2ad-5aaf-83c7-31e919129bbf@nbd.name>
Date:   Thu, 17 Dec 2020 14:01:03 +0100
Message-ID: <871rfobn8w.fsf@toke.dk>
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Felix Fietkau <nbd@nbd.name> writes:

> On 2020-12-17 12:54, Toke H=C3=B8iland-J=C3=B8rgensen wrote:
>> Felix Fietkau <nbd@nbd.name> writes:
>>=20
>>> Depending on the source, a hardware calculated hash may not provide the
>>> same level of collision resistance.
>>=20
>> This seems like it would have performance implications?
>>=20
>> Also, this can potentially discard information from tunnels that
>> preserve the hash before encapsulation (we added support for this to
>> Wireguard which had some nice effects on queueing of encapsulated
>> traffic).
> If the hash was calculated in software using the flow dissector, it will
> be preserved, even if it went through a few virtual interfaces.
> The only hashes discarded are hardware generated ones.

Yeah, but I was thinking something like:

Packet comes in with HW hash -> gets encapsulated (preserving the hash)
-> gets to mac80211 which discards the HW hash. So now you're replacing
a (possibly bad-quality) HW hash with a software hash of the *outer*
encapsulation header...

-Toke