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[121.45.221.81]) by smtp.gmail.com with ESMTPSA id 79sm331948pfz.23.2020.03.25.18.26.57 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 25 Mar 2020 18:27:00 -0700 (PDT) Subject: Re: [PATCH 1/2] dma-mapping: add a dma_ops_bypass flag to struct device To: Christoph Hellwig References: <20200320141640.366360-1-hch@lst.de> <20200320141640.366360-2-hch@lst.de> <2f31d0dd-aa7e-8b76-c8a1-5759fda5afc9@ozlabs.ru> <20200323083705.GA31245@lst.de> <20200323085059.GA32528@lst.de> <87sghz2ibh.fsf@linux.ibm.com> <20200323172256.GB31269@lst.de> <20200324075402.GJ23447@lst.de> <41975da3-3a4a-fc3c-2b90-8d607cf220e6@ozlabs.ru> <20200325083740.GC21605@lst.de> From: Alexey Kardashevskiy Autocrypt: addr=aik@ozlabs.ru; keydata= mQINBE+rT0sBEADFEI2UtPRsLLvnRf+tI9nA8T91+jDK3NLkqV+2DKHkTGPP5qzDZpRSH6mD EePO1JqpVuIow/wGud9xaPA5uvuVgRS1q7RU8otD+7VLDFzPRiRE4Jfr2CW89Ox6BF+q5ZPV /pS4v4G9eOrw1v09lEKHB9WtiBVhhxKK1LnUjPEH3ifkOkgW7jFfoYgTdtB3XaXVgYnNPDFo 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Sender: "iommu" On 25/03/2020 19:37, Christoph Hellwig wrote: > On Wed, Mar 25, 2020 at 03:51:36PM +1100, Alexey Kardashevskiy wrote: >>>> This is for persistent memory which you can DMA to/from but yet it does >>>> not appear in the system as a normal memory and therefore requires >>>> special handling anyway (O_DIRECT or DAX, I do not know the exact >>>> mechanics). All other devices in the system should just run as usual, >>>> i.e. use 1:1 mapping if possible. >>> >>> On other systems (x86 and arm) pmem as long as it is page backed does >>> not require any special handling. This must be some weird way powerpc >>> fucked up again, and I suspect you'll have to suffer from it. >> >> >> It does not matter if it is backed by pages or not, the problem may also >> appear if we wanted for example p2p PCI via IOMMU (between PHBs) and >> MMIO might be mapped way too high in the system address space and make >> 1:1 impossible. > > How can it be mapped too high for a direct mapping with a 64-bit DMA > mask? The window size is limited and often it is not even sparse. It requires an 8 byte entry per an IOMMU page (which is most commonly is 64k max) so 1TB limit (a guest RAM size) is a quite real thing. MMIO is mapped to guest physical address space outside of this 1TB (on PPC). -- Alexey _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu