linux-kernel - Re: [PATCH net-next v2 02/12] iavf: kill "legacy-rx" for good

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Date:   Tue, 30 May 2023 18:22:07 +0200
From:   Alexander Lobakin <aleksander.lobakin@...el.com>
To:     Alexander H Duyck <alexander.duyck@...il.com>
CC:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Michal Kubiak <michal.kubiak@...el.com>,
        Larysa Zaremba <larysa.zaremba@...el.com>,
        "Jesper Dangaard Brouer" <hawk@...nel.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Christoph Hellwig <hch@....de>,
        Paul Menzel <pmenzel@...gen.mpg.de>, <netdev@...r.kernel.org>,
        <intel-wired-lan@...ts.osuosl.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next v2 02/12] iavf: kill "legacy-rx" for good

From: Alexander H Duyck <alexander.duyck@...il.com>
Date: Tue, 30 May 2023 08:29:10 -0700

> On Thu, 2023-05-25 at 14:57 +0200, Alexander Lobakin wrote:
>> Ever since build_skb() became stable, the old way with allocating an skb
>> for storing the headers separately, which will be then copied manually,
>> was slower, less flexible and thus obsolete.

[...]

> The "legacy-rx" was never about performance. It was mostly about
> providing a fall back in the event of an unexpected behavior. Keep in
> mind that in order to enable this we are leaving the page mapped and
> syncing it multiple times. In order to enable support for this we had
> to add several new items that I had deemed to be a bit risky such as
> support for DMA pages that were synced by the driver instead of on
> map/unmap and the use of the build_skb logic.

And now pretty much every driver doesn't do map/unmap each time and
keeps its mapping for a very long time. And now Chris noticed that and
says it's not supposed to be used like that :D

> 
> My main concern was that if we ever ran into  header corruption we
> could switch this on and then the pages would only be writable by the
> device.

[...]

> Since it has been about 6 years without any issues I would say we are
> safe to remove it.
> 
> Reviewed-by: Alexander Duyck <alexanderduyck@...com>

Thanks!
Olek