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[47.55.120.4]) by smtp.gmail.com with ESMTPSA id af79cd13be357-7e67f5cd63fsm689744985a.39.2025.08.05.08.36.07 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 05 Aug 2025 08:36:07 -0700 (PDT) Received: from jgg by wakko with local (Exim 4.97) (envelope-from ) id 1ujJhu-00000001YWx-3rSt; Tue, 05 Aug 2025 12:36:06 -0300 Date: Tue, 5 Aug 2025 12:36:06 -0300 From: Jason Gunthorpe To: Matthew Wilcox Cc: Robin Murphy , Marek Szyprowski , Christoph Hellwig , Leon Romanovsky , Jonathan Corbet , Madhavan Srinivasan , Michael Ellerman , Nicholas Piggin , Christophe Leroy , Joerg Roedel , Will Deacon , "Michael S. Tsirkin" , Jason Wang , Xuan Zhuo , Eugenio =?utf-8?B?UMOpcmV6?= , Alexander Potapenko , Marco Elver , Dmitry Vyukov , Masami Hiramatsu , Mathieu Desnoyers , =?utf-8?B?SsOpcsO0bWU=?= Glisse , Andrew Morton , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, iommu@lists.linux.dev, virtualization@lists.linux.dev, kasan-dev@googlegroups.com, linux-trace-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH 0/8] dma-mapping: migrate to physical address-based API Message-ID: <20250805153606.GR26511@ziepe.ca> References: <35df6f2a-0010-41fe-b490-f52693fe4778@samsung.com> <20250627170213.GL17401@unreal> <20250630133839.GA26981@lst.de> <69b177dc-c149-40d3-bbde-3f6bad0efd0e@samsung.com> <20250803155906.GM26511@ziepe.ca> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Queue-Id: 5978720007 X-Rspamd-Server: rspam06 X-Stat-Signature: hqnbnognkrpsospd3yxztdcjrkeeoghi X-HE-Tag: 1754408169-812292 X-HE-Meta: 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 oNcEiif5 Fn0NJi1Qt3OXIhkU6Z4va7F3pnMI7vGWJBoTIBUu4P7my1BgFcSTJvVULThJbdh1AjDCQ6Kmq4Lm2x2OzTncr+5SlAow4uH7G3FEcILKdqpIDX+OP3+m+qwci6OEtEv4P8ShMhrDXfZL5gGrviPJXZCEzz2QcZv8Hrb3t+IKIDN8zwaYeLtDK0oOABXw9BB4gtN/d31kGt+azVUtPhKiOEIT7LrcRZMdQj4bGYRBYn/hmyW9m7K84IQ5KuIJbwrY5tgxgm1tTeOFJ7TRU89Fy7EBzNCPJotjVQmzo X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Aug 04, 2025 at 04:37:56AM +0100, Matthew Wilcox wrote: > On Sun, Aug 03, 2025 at 12:59:06PM -0300, Jason Gunthorpe wrote: > > Matthew, do you think it makes sense to introduce types to make this > > clearer? We have two kinds of values that a phys_addr_t can store - > > something compatible with kmap_XX_phys(), and something that isn't. > > I was with you up until this point. And then you said "What if we have > a raccoon that isn't a raccoon" and my brain derailed. I though it was clear.. kmap_local_pfn(phys >> PAGE_SHIFT) phys_to_virt(phys) Does not work for all values of phys. It definately illegal for non-cachable MMIO. Agree? There is a subset of phys that is cachable and has struct page that is usable with kmap_local_pfn()/etc phys is always this: > - CPU untranslated. This is the "physical" address. Physical address > 0 is what the CPU sees when it drives zeroes on the memory bus. But that is a pure HW perspective. It doesn't say which of our SW APIs are allowed to use this address. We have callchains in DMA API land that want to do a kmap at the bottom. It would be nice to mark the whole call chain that the phys_addr being passed around is actually required to be kmappable. Because if you pass a non-kmappable MMIO backed phys it will explode in some way on some platforms. > > We clearly have these two different ideas floating around in code, > > page tables, etc. > No. No, we don't. I've never heard of this asininity before. Welcome to the fun world of cachable and non-cachable memory. Consider, today we can create struct pages of type MEMORY_DEVICE_PCI_P2PDMA for non-cachable MMIO. I think today you "can" use kmap to establish a cachable mapping in the vmap. But it is *illegal* to establish a cachable CPU mapping of MMIO. Archs are free to MCE if you do this - speculative cache line load of MMIO can just error in HW inside the interconnect. So, the phys_addr is always a "CPU untranslated physical address" but the cachable/non-cachable cases, or DRAM vs MMIO, are sometimes semantically very different things for the SW! Jason