On Mon, Jul 03, 2017 at 07:11:28PM +0100, Russell King - ARM Linux wrote: > On Mon, Jul 03, 2017 at 08:40:31AM -0500, Bjorn Helgaas wrote: > > The problem is serializing vs. memory accesses, since they don't use > > any wrappers. However, they are ioremapped(), so it's at least > > conceivable that another solution would be to use VM to trap those > > accesses. I'm not a VM person, so I don't know whether that's > > feasible in Linux. > > Bjorn, > > You're forgetting that MMIO (iow, memory returned by ioremap()) must > be accessed through the appropriate accessors, and must not be > directly dereferenced in C. (We do have buggy drivers that do that > but they are buggy, and in many cases are getting attention to fix > that.) Oh, you're right, thank you! I guess you're referring to readb() and friends. I haven't found an actual prohibition on directly dereferencing addresses returned from ioremap(), but Documentation/driver-api/device-io.rst is clear that they're suitable for passing to readb(), etc. I recently told someone else my mistaken idea that ioremap() must return a valid virtual address. I wish I remembered who it was, so I could correct that. Documentation/DMA-API-HOWTO.txt also suggests that ioremap() returns a virtual address -- I think I wrote that, and maybe that virtual address reference should be tweaked a bit. Another wrinkle is that the pci_mmap_resource() interface is exposed via sysfs and allows direct userspace mmap of PCI MMIO resources. In that case, there is no accessor available. I wonder if we need some way to disable this mmap when readb() is non-trivial. Bjorn
