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[35.189.192.250]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4a19f249692sm172738315e9.3.2026.10.12.07.49.26 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 12 Oct 2026 07:49:26 -0700 (PDT) Date: Mon, 12 Oct 2026 14:49:22 +0000 From: Mostafa Saleh To: "David Hildenbrand (Arm)" Cc: Luigi Rizzo , Luigi Rizzo , Joerg Roedel , Will Deacon , Robin Murphy , Christoph Hellwig , Marek Szyprowski , Andrew Morton , Vlastimil Babka , "David S . Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Greg Kroah-Hartman , "Rafael J . Wysocki" , Danilo Krummrich , Jonathan Corbet , Jesper Dangaard Brouer , Ilias Apalodimas , Willem de Bruijn , Kuniyuki Iwashima , Joshua Washington , Harshitha Ramamurthy , Saeed Mahameed , Tariq Toukan , Tony Nguyen , Przemek Kitszel , Alexander Lobakin , Michael Chan , Pavan Chebbi , iommu@lists.linux.dev, netdev@vger.kernel.org, linux-mm@kvack.org, driver-core@lists.linux.dev, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [RFC: DMA_PMD 00/22] DMA_PMD: PMD_SIZE-backed IO buffer pools Message-ID: References: <20261003212241.3432303-1-lrizzo@google.com> <0cc1cac1-a500-489b-9c1b-b49598b5550b@kernel.org> Precedence: bulk X-Mailing-List: driver-core@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <0cc1cac1-a500-489b-9c1b-b49598b5550b@kernel.org> On Mon, Oct 12, 2026 at 03:51:39PM +0200, David Hildenbrand (Arm) wrote: > On 10/3/26 23:22, Luigi Rizzo wrote: > > Here is a subsystem called DMA_PMD on which I would like feedback on > > architecture, possible enhancements, or kernel components that could be > > reused to avoid duplication. > > Here is a feedback to a subsystem called DMA_PMD ;) > > Why is basing this on PMDs (and embedding it in the name) the right approach? > > On architctures with large PMDs (512 MiB on arm64 with 64k), would we really > want to use 512 MiB pages, or rather something smaller (e.g., 2 MiB) that might > possibly also have TLB benefits (cont-pte)? +1 I raised the same point in the LPC talk. AFAIK, this series tries to solve both IOTLB pressure and CoCo bouncing at the same time which might be too strict; as for CoCo there is no actual IOMMU, so there is no sizing constraint. Also to reduce bouncing, I think it is sufficient to use the SWIOTLB instead of having a new pool + recycling, the tricky part is how to abstract that to client drivers as this would be needed beyond SKB. Thanks, Mostafa > > -- > Cheers, > > David >