Re: [PATCH v2 6/6] x86, pat: Update documentation for WT changes

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

On Mon, 2014-09-15 at 18:22 -0700, Andy Lutomirski wrote:
> On Mon, Sep 15, 2014 at 2:19 PM, Toshi Kani <toshi.kani@hp.com> wrote:
> > On Wed, 2014-09-10 at 15:34 -0600, Toshi Kani wrote:
> >> On Wed, 2014-09-10 at 13:29 -0700, Andy Lutomirski wrote:
> >> > On Wed, Sep 10, 2014 at 1:12 PM, Toshi Kani <toshi.kani@hp.com> wrote:
> >> > > On Wed, 2014-09-10 at 11:30 -0700, Andy Lutomirski wrote:
> >> > >> On Wed, Sep 10, 2014 at 9:51 AM, Toshi Kani <toshi.kani@hp.com> wrote:
> >> > >> > +Drivers may map the entire NV-DIMM range with ioremap_cache and then change
> >> > >> > +a specific range to wt with set_memory_wt.
> >> > >>
> >> > >> That's mighty specific :)
> >> > >
> >> > > How about below?
> >> > >
> >> > > Drivers may use set_memory_wt to set WT type for cached reserve ranges.
> >> >
> >> > Do they have to be cached?
> >>
> >> Yes, set_memory_xyz only supports WB->type->WB transition.
> >>
> >> > How about:
> >> >
> >> > Drivers may call set_memory_wt on ioremapped ranges.  In this case,
> >> > there is no need to change the memory type back before calling
> >> > iounmap.
> >> >
> >> > (Or only on cached ioremapped ranges if that is, in fact, the case.)
> >>
> >> Sounds good.  Yes, I will use cashed ioremapped ranges.
> >
> > Well, testing "no need to change the memory type back before calling
> > iounmap" turns out to be a good test case.  I realized that
> > set_memory_xyz only works properly for RAM.  There are two problems for
> > using this interface for ioremapped ranges.
> >
> > 1) set_memory_xyz calls reserve_memtype() with __pa(addr).  However,
> > __pa() translates the addr into a fake physical address when it is an
> > ioremapped address.
> >
> > 2) reserve_memtype() does not work for set_memory_xyz.  For RAM, the WB
> > state is managed untracked.  Hence, WB->new->WB is not considered as a
> > conflict.  For ioremapped ranges, WB is tracked in the same way as other
> > cache types.  Hence, WB->new is considered as a conflict.
> >
> > In my previous testing, 2) was undetected since 1) led using a fake
> > physical address which was not tracked for WB.  This made ioremapped
> > ranges worked just like RAM. :-(
> >
> > Anyway, 1) can be fixed by using slow_virt_to_phys() instead of __pa().
> > set_memory_xyz is already slow, but this makes it even slower, though.
> >
> > For 2), WB has to be continuously tracked in order to detect aliasing,
> > ex. ioremap_cache and ioremap to a same address.  So, I think
> > reserve_memtype() needs the following changes:
> >  - Add a new arg to see if an operation is to create a new mapping or to
> > change cache attribute.
> >  - Track overlapping maps so that cache type change to an overlapping
> > range can be detected and failed.
> >
> > This level of changes requires a separate set of patches if we pursue to
> > support ioremapped ranges.  So, I am considering to take one of the two
> > options below.
> >
> > A) Drop the patch for set_memory_wt.
> >
> > B) Keep the patch for set_memory_wt, but document that it fails with
> > -EINVAL and its use is for RAM only.
> >
> 
> I vote A.  I see no great reason to add code that can't be used.  Once
> someone needs this ability, they can add it :)

Agreed.  I will drop the patch for now.  Since _PGMT_WB does not seem to
be used for tracking WB, we might be able to use this bit for WT.  But I
need to look at the code more carefully for sure.

> It's too bad that ioremap is called ioremap and not iomap.  Otherwise
> the natural solution would be to add a different function call
> ioremap_wt that's like set_memory_wt but for ioremap ranges.  Calling
> it ioreremap_wt sounds kind of disgusting :)

:)

Thanks,
-Toshi

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