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b=RGeCakC/vm3j1lGVc0QsHjNhlxLL2gR4k5Xp7Xih5V0wRu/E7L3jfbzjuTAhfGC3o sFn7WizIBbY7BsmMESWwoIqWg+sVCjqVmfLZfVF0HeGo2S8Qpz1MAbmW4wWuhxtYE3 DmghrCQLA4zd1RDZJ8D5bHVuhr4Xk+gM/UF6+zHOHOyXdun6WfctItNst70WTGiHLc I6s84loPj2TV2OpzCFwjcsNdGDDHOrbpK5UmyV7ZU/qwjf8K/OGsvP/kelTxx0s+nc QEp5I2evFlGfKK4Q4Xrsaq31r/Ylc5w87cNKhapxmDGO+5I4yep3ub5H1i+dOrdL9H HU+98aBGVIQOg== X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200910_192950_634188_41537C80 X-CRM114-Status: GOOD ( 22.14 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Lorenzo Pieralisi , Leon Romanovsky , linux-pci@vger.kernel.org, Bjorn Helgaas , catalin.marinas@arm.com, will@kernel.org, linux-arm-kernel@lists.infradead.org, Clint Sbisa Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Fri, Sep 11, 2020 at 07:46:47AM +1000, Benjamin Herrenschmidt wrote: > On Thu, 2020-09-10 at 14:10 -0300, Jason Gunthorpe wrote: > > Can you explain what this actually does on ARM? > > > > Can it ever speculate loads across page boundaries, or speculate > > loads > > that never exist in the program? ie will we get random unpredicable > > MemRds? > > Probably, at least on powerpc you will as well, that's the only way to > get write combine. If I remove the PROT_READ in the user space mmap will it block it? Read TLPs are not harmful but I suspect they would cause an undesirable random performance anomaly. > > Does it/could it "combine writes"? > > I assume so for ARM, definitely for powerpc. Various IBM PPC chips I know work, we do test that. > > > That's why I looped you in - that's what worries me about > > > "enabling" > > > arch_can_pci_mmap_wc() on arm64. If we enable it and we have perf > > > regressions that's not OK. > > > > > > Or we *can* enable arch_can_pci_mmap_wc() but force the mellanox > > > driver (or more broadly all drivers following this message push > > > semantics) to use "something else" for WC detection. > > > > arch_can_pci_mmap_wc() really only controls the sysfs resource file > > and it seems very unclear who in userspace uses that these days. > > dpdk under some circumstances afaik. And something gross for DMA then? Not sure dpdk is useful without DMA. Why not use CONFIG_VFIO_NOIOMMU for such a non-secure thing? > > vfio is now the right way to do that stuff. I don't see an obvious > > way to get WC memory in VFIO though... > > Which would be a performance issue on a number of things I suppose... Almost nothing uses pci_iomap_wc(), so I'd be surpried if userspace DPDK was an important user when an in-kernel driver for the same HW doesn't use it? Jason _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel