On Thu, 2020-09-10 at 20:29 -0300, Jason Gunthorpe wrote: > > 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? No. powerpc at least doesn't have write-only mappings. > Read TLPs are not harmful but I suspect they would cause an > undesirable random performance anomaly. I suspect in practice you wont get them esp. if the code has barriers but ... it's allowed by the architecture. > > > 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? Clint, can you elaborate on the use case ? > > > 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? Hard to know how uses those files out there but I don't like arm not providing what pretty much all relevant archs do provide since the semantics afaik aren't that different. Yes, "write combine" isn't a good name.... The goal is to get WC but it comes with the whole package on several archs. We don't even have a reasonnable definition of the semantics of readl/writel on a WC mapping (hint: on powerpc the barriers in them will prevent WC even on a WC mapping) nor of what barriers might work and how on such a mapping. I tried a while ago and ... ugh. Cheers, Ben.
