Re: Overlapping ioremap() calls, set_memory_*() semantics

[Toshi Kani <toshi.kani@hpe.com>]

On Wed, 2016-03-16 at 02:45 +0100, Luis R. Rodriguez wrote:
> On Fri, Mar 11, 2016 at 03:13:52PM -0700, Toshi Kani wrote:
> > On Wed, 2016-03-09 at 10:15 +0100, Ingo Molnar wrote:
> > > * Toshi Kani <toshi.kani@hpe.com> wrote:
> > > 
> > > > On Tue, 2016-03-08 at 13:16 +0100, Ingo Molnar wrote:
> > > > > * Toshi Kani <toshi.kani@hpe.com> wrote:
> > > > > 
 :
> > > > Did you mean 'aliased' or 'aliased with different cache attribute'?
> > > >  The former check might be too strict.
> > > 
> > > I'd say even 'same attribute' aliasing is probably relatively rare.
> > > 
> > > And 'different but compatible cache attribute' is in fact more of a
> > > sign that the driver author does the aliasing for a valid _reason_:
> > > to have two different types of access methods to the same piece of
> > > physical address space...
> > 
> > Right.  So, if we change to fail ioremap() on aliased cases, it'd be
> > easier to start with the different attribute case first.  This case
> > should be rare enough that we can manage to identify such callers and
> > make them use a new API as necessary.  If we go ahead to fail any
> > aliased cases, it'd be challenging to manage without a regression or
> > two.
> 
> From my experience on the ioremap_wc() crusade, I found that the need for
> aliasing with different cache types would have been needed in only 3
> drivers. For these 3, the atyfb driver I did the proper split in MMIO and
> framebuffer, but that was significant work.  I did this work to demo and
> document such work. It wasn't easy. For other two, ivtv and ipath we left
> as requiring "nopat" to be used. The ipath driver is on its way out of
> the kenrel now through staging, and ivtv, well I am not aware of single
> human being claiming to use it. The architecture of ivtv actually
> prohibits us from ever using PAT for write-combining on the framebuffer
> as the firmware is the only one who knows the write-combining area and
> hides it from us.

At glace, there are 863 references to ioremap(), 329 references to
ioremap_nocache(), and only 68 references to ioremap_wc() on x86.  There
are many more ioremap callers with UC mappings than WC mappings, and it is
hard to say that they never get aliased.

> We might be able to use tools like Coccinelle to perhaps hunt for
> the use of aliasing on drivers with different cache attribute types
> to do a full assessment but I really think that will be really hard
> to accomplish.
> 
> If we can learn anything from the ioremap_wc() crusade I'd say its that
> the need for aliasing with different cache types obviously implies we
> should disable such drivers with PAT as what we'd really need is a proper
> split in maps, but history shows the split can be really hard. It sounded
> like you guys were confirming we currently do not allow for aliasing with
> different attributes on x86, is that the case for all architectures?
> 
> If aliasing with different cache attributes is not allowed for x86 and
> if its also rare for other architectures that just leaves the hunt for
> valid aliasing uses. That still may be hard to hunt for, but I also
> suspect it may be rare.

Yes, I'd fail the different cache attribute case if we are to place more
strict check.

 :
> > 
> > I think the "set_memory_" prefix implies that their target is regular
> > memory only.
> 
> I did not find any driver using set_memory_wc() on MMIO, its a good thing
> as that does not work it seems even if it returns no error.  I'm not sure
> of the use of other set_memory_*() on MMIO but I would suspect its not
> used. A manual hunt may suffice to rule these out.

It's good to know that you did not find any case on MMIO.  The thing is,
set_memory_wc() actually works on MMIO today... This is because __pa()
returns a bogus address, which skips the alias check in the memtype.

> I guess what I'm trying to say is I am not sure we have a need for
> set_cache_attr_*() APIs, unless of course we find such valid use.
> 
> > > And at that point we could definitely argue that set_cache_attr_*()
> > > APIs should probably generate a warning for _RAM_, because they
> > > mostly make sense for MMIO type of physical addresses, right? Regular
> > > RAM should always be WB.
> > > 
> > > Are there cases where we change the caching attribute of RAM for
> > > valid reasons, outside of legacy quirks?
> > 
> > ati_create_page_map() is one example that it gets a RAM page
> > by __get_free_page(), and changes it to UC by calling set_memory_uc().
> 
> Should we instead have an API that lets it ask for RAM and of UC type?
> That would seem a bit cleaner. BTW do you happen to know *why* it needs
> UC RAM types?

This RAM page is then shared between graphic card and CPU.  I think this is
because graphic card cannot snoop the cache.

> > 
 :
> > > >  - It only supports attribute transition of {WB -> NewType -> WB}
> > > > for RAM.  RAM is tracked differently that WB is treated as "no
> > > > map".  So, this transition does not cause a conflict on RAM.  This
> > > > will causes a conflict on MMIO when it is tracked correctly.   
> > > 
> > > That looks like a bug?
> > 
> > This is by design since set_memory_xx was introduced for RAM only.  If
> > we extend it to MMIO, then we need to change how memtype manages MMIO.
> 
> I'd be afraid to *want* to support this on MMIO as I would only expect
> hacks from drivers.

Agreed, with the hope that they are not used on MMIO already...

Thanks,
-Toshi