linux-kernel - Re: [PATCH net] xsk: remove cheap

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Date:   Mon, 29 Jun 2020 15:52:51 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Björn Töpel <bjorn.topel@...el.com>,
        Christoph Hellwig <hch@....de>
Cc:     Björn Töpel <bjorn.topel@...il.com>,
        netdev@...r.kernel.org, davem@...emloft.net,
        konrad.wilk@...cle.com, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
        maximmi@...lanox.com, magnus.karlsson@...el.com,
        jonathan.lemon@...il.com
Subject: Re: [PATCH net] xsk: remove cheap_dma optimization

On 6/28/20 7:16 PM, Björn Töpel wrote:
> On 2020-06-27 09:04, Christoph Hellwig wrote:
>> On Sat, Jun 27, 2020 at 01:00:19AM +0200, Daniel Borkmann wrote:
>>> Given there is roughly a ~5 weeks window at max where this removal could
>>> still be applied in the worst case, could we come up with a fix / proposal
>>> first that moves this into the DMA mapping core? If there is something that
>>> can be agreed upon by all parties, then we could avoid re-adding the 9%
>>> slowdown. :/
>>
>> I'd rather turn it upside down - this abuse of the internals blocks work
>> that has basically just missed the previous window and I'm not going
>> to wait weeks to sort out the API misuse.  But we can add optimizations
>> back later if we find a sane way.
> 
> I'm not super excited about the performance loss, but I do get
> Christoph's frustration about gutting the DMA API making it harder for
> DMA people to get work done. Lets try to solve this properly using
> proper DMA APIs.

Ok, fair enough, please work with DMA folks to get this properly integrated and
restored then. Applied, thanks!