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[47.54.130.67]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-51c4202366csm13366551cf.30.2026.07.03.04.46.56 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 03 Jul 2026 04:46:56 -0700 (PDT) Received: from jgg by wakko with local (Exim 4.97) (envelope-from ) id 1wfcMB-00000007ngp-0vd7; Fri, 03 Jul 2026 08:46:55 -0300 Date: Fri, 3 Jul 2026 08:46:55 -0300 From: Jason Gunthorpe To: Alexey Kardashevskiy Cc: "Aneesh Kumar K.V (Arm)" , iommu@lists.linux.dev, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-coco@lists.linux.dev, Robin Murphy , Marek Szyprowski , Will Deacon , Marc Zyngier , Steven Price , Suzuki K Poulose , Catalin Marinas , Jiri Pirko , Mostafa Saleh , Petr Tesarik , Dan Williams , Xu Yilun , linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, Madhavan Srinivasan , Michael Ellerman , Nicholas Piggin , "Christophe Leroy (CS GROUP)" , Alexander Gordeev , Gerald Schaefer , Heiko Carstens , Vasily Gorbik , Christian Borntraeger , Sven Schnelle , x86@kernel.org, Jiri Pirko , Michael Kelley , "Cheloha, Scott" Subject: Re: [PATCH v6 03/20] dma-direct: use DMA_ATTR_CC_SHARED in alloc/free paths Message-ID: <20260703114655.GW7525@ziepe.ca> References: <20260604083959.1265923-1-aneesh.kumar@kernel.org> <20260604083959.1265923-4-aneesh.kumar@kernel.org> <845d0c8a-6d51-47aa-8e0b-8381e733444a@amd.com> <20260617154101.GE3577091@ziepe.ca> <25155bd6-4348-4aa8-ba70-0a882fc84db9@amd.com> <20260630160242.GI7525@ziepe.ca> <98800d21-18f6-44d6-9ba0-da9b77567d85@amd.com> <20260702144729.GP7525@ziepe.ca> <58bb92a9-c5ea-47a3-b19a-934573f15305@amd.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <58bb92a9-c5ea-47a3-b19a-934573f15305@amd.com> On Fri, Jul 03, 2026 at 08:15:49PM +1000, Alexey Kardashevskiy wrote: > On 3/7/26 00:47, Jason Gunthorpe wrote: > > On Thu, Jul 02, 2026 at 10:25:16AM +1000, Alexey Kardashevskiy wrote: > > > > > > > not externally available so I'll have to trick the DMA layer into > > > > > using SWIOTLB (which is still all shared, right?) as I specifically > > > > > want to skip page conversions. Setting low DMA mask won't guarantee > > > > > that the DMA layer won't allocate a page outside of SWIOTLB and > > > > > convert it. Manually do > > > > > > > > Why so particular? > > > > > > aahhh I missed "pre-". I need a way to get pre-shared pages for my > > > sev-guest activities. > > > > It sounds to me like you don't, what you are worried about is > > optimizing the dma_alloc_coherent flow to avoid fragmentation and all > > CC architetures require this optimization. > > It is not about fragmentation, it is about unwanted page state > changes when few pages are shared for a very short time. That's really basically the same thing. The responsiblity for optimizing the shared/private conversion lies with the DMA API not the callers. > When I needed these pages, I could: get a private page, convert, > (possibly trigger RMP and IO PDE smashing if that 4K is backed with > 2M), do the VM<->HV communication, convert back to private (and > possibly unsmash the page). IMHO it would be better to use something like dma_pool to amortize the coherent allocations instead of trying to hack with swiotlb. Jason