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Date:   Thu, 18 May 2023 15:26:15 +0200
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Subject: Re: [PATCH net-next 06/11] net: page_pool: avoid calling no-op
 externals when possible
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To:     Christoph Hellwig <hch@lst.de>
CC:     "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Maciej Fijalkowski <maciej.fijalkowski@intel.com>,
        Magnus Karlsson <magnus.karlsson@intel.com>,
        Michal Kubiak <michal.kubiak@intel.com>,
        Larysa Zaremba <larysa.zaremba@intel.com>,
        "Jesper Dangaard Brouer" <hawk@kernel.org>,
        Ilias Apalodimas <ilias.apalodimas@linaro.org>,
        <netdev@vger.kernel.org>, <intel-wired-lan@lists.osuosl.org>,
        <linux-kernel@vger.kernel.org>
References: <20230516161841.37138-1-aleksander.lobakin@intel.com>
 <20230516161841.37138-7-aleksander.lobakin@intel.com>
 <20230517081458.GA32192@lst.de>
From:   Alexander Lobakin <aleksander.lobakin@intel.com>
In-Reply-To: <20230517081458.GA32192@lst.de>
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From: Christoph Hellwig <hch@lst.de>
Date: Wed, 17 May 2023 10:14:58 +0200

> So while this looks ok, Eric Dumazet had looked into some optimization
> for this touching the core code, and promised me to come up with an
> even better version a while ago.  Eric, what's the state of your
> optimizations for no-op DMA syncs?
> 

Hmm, his last proposals were for avoiding indirect calls when IOMMU is
on, but in fact we don't need to synchronize stuff -- DMA IOMMU on
x86_64 also usually doesn't synchronize anything, but you don't know
that prior to doing an indirect call and dma_need_sync() won't help.
I was thinking of adding .dma_need_sync() callback to DMA ops, which
could also be called once only on page allocation, like in this patch.

Also want to hear how it goes for Eric :)

Thanks,
Olek