On Thu, 2015-07-09 at 03:40 +0200, Luis R. Rodriguez wrote:
On Tue, Jul 07, 2015 at 05:10:58PM -0600, Toshi Kani wrote:
> On Tue, 2015-07-07 at 18:07 +0200, Luis R. Rodriguez wrote:
> > On Tue, Jul 07, 2015 at 11:13:30AM +0100, Russell King - ARM
> > Linux
> > wrote:
> > > On ARM, we (probably) have a lot of cases where ioremap() is
> > > used
> > > multiple
> > > times for the same physical address space, so we shouldn't rule
> > > out
> > > having
> > > multiple mappings of the same type.
> > Why is that done? Don't worry if you are not sure why but only
> > speculate of the
> > practice's existence (sloppy drivers or lazy driver developers).
> > FWIW
> > for x86
> > IIRC I ended up concluding that overlapping ioremap() calls with
> > the
> > same type
> > would work but not if they differ in type. Although I haven't
> > written a
> > grammer rule to hunt down overlapping ioremap() I suspected its
> > use
> > was likely
> > odd and likely should be reconsidered. Would this be true for ARM
> > too
> > ? Or are
> > you saying this should be a feature ? I don't expect an answer
> > now
> > but I'm
> > saying we *should* all together decide on this, and if you're
> > inclined to
> > believe that this should ideally be avoided I'd like to hear
> > that. If
> > you feel
> > strongly though this should be a feature I would like to know
> > why.
> There are multiple mapping interfaces, and overlapping can happen
> them as well. For instance, remap_pfn_range() (and
> io_remap_pfn_range(), which is the same as remap_pfn_range() on
> creates a mapping to user space. The same physical ranges may be
> mapped to kernel and user spaces. /dev/mem is one example that may
> create a user space mapping to a physical address that is already
> mapped with ioremap() by other module.
Thanks for the feedback. The restriction seems to be differing cache
requirements, other than this, are there any other concerns ? For
we completley happy with aliasing so long as cache types match
expect no architecture would want cache types to differ when
should differ then I think would just be how to verify this and it
we may be doing this for all architectures.
Even for userspace we seem to be covered -- we enable userspace
mmap() calls to
get their mapped space with a cache type, on the kernel we'd say use
pgprot_writecombine() on the vma->vm_page_prot prior to the
io_remap_pfn_range() -- that maps to remap_pfn_range() on x86 and as
that checks cache type via reserve_memtype() -- but only on x86...
Other than this differing cache type concern are we OK with aliasing
userspace all the time ?
If we want to restrict aliasing either for the kernel or userspace
we might be able to do it, I just want to know if we want to or not
Yes, we allow to create multiple mappings to a same physical page as
long as their cache type is the same. There are multiple use-cases
that depend on this ability.
> pmem and DAX also create mappings to the same NVDIMM ranges.
> vm_insert_mixed(), which is particularly a problematic since
> vm_insert_mixed() does not verify aliasing. ioremap() and
> call reserve_memtype() to verify aliasing on x86.
> reserve_memtype() is
> x86-specific and there is no arch-generic wrapper for such check.
As clarified by Matthew Wilcox via commit d92576f1167cacf7844 ("dax:
work correctly with virtual aliasing caches") caches are virtually
some architectures, it seems it should be possible to fix this for
I simply described this DAX case as an example of how two modules might
request different cache types. Yes, we should be able to fix this
> I think DAX could get a cache type from pmem to keep them in
pmem is x86 specific right now, are other folks going to expose
similar ? Otherwise we seem to only be addressing these deep concerns
x86 so far.
pmem is a generic driver and is not x86-specific.