Re: [PATCH v9 2/3] PCI: Add tango PCIe host bridge support

[Bjorn Helgaas <helgaas@kernel.org>]

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