Re: [PATCH v10 12/12] drivers/block/pmem: Map NVDIMM with ioremap_wt()

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

On Fri, 2015-05-29 at 12:34 -0700, Dan Williams wrote:
> On Fri, May 29, 2015 at 11:32 AM, Toshi Kani <toshi.kani@hp.com> wrote:
> > On Fri, 2015-05-29 at 11:19 -0700, Dan Williams wrote:
> >> On Fri, May 29, 2015 at 8:03 AM, Toshi Kani <toshi.kani@hp.com> wrote:
> >> > On Fri, 2015-05-29 at 07:43 -0700, Dan Williams wrote:
> >> >> On Fri, May 29, 2015 at 2:11 AM, Borislav Petkov <bp@alien8.de> wrote:
> >> >> > On Wed, May 27, 2015 at 09:19:04AM -0600, Toshi Kani wrote:
> >> >> >> The pmem driver maps NVDIMM with ioremap_nocache() as we cannot
> >  :
> >> >> >> -     pmem->virt_addr = ioremap_nocache(pmem->phys_addr, pmem->size);
> >> >> >> +     pmem->virt_addr = ioremap_wt(pmem->phys_addr, pmem->size);
> >> >> >>       if (!pmem->virt_addr)
> >> >> >>               goto out_release_region;
> >> >> >
> >> >> > Dan, Ross, what about this one?
> >> >> >
> >> >> > ACK to pick it up as a temporary solution?
> >> >>
> >> >> I see that is_new_memtype_allowed() is updated to disallow some
> >> >> combinations, but the manual seems to imply any mixing of memory types
> >> >> is unsupported.  Which worries me even in the current code where we
> >> >> have uncached mappings in the driver, and potentially cached DAX
> >> >> mappings handed out to userspace.
> >> >
> >> > is_new_memtype_allowed() is not to allow some combinations of mixing of
> >> > memory types.  When it is allowed, the requested type of ioremap_xxx()
> >> > is changed to match with the existing map type, so that mixing of memory
> >> > types does not happen.
> >>
> >> Yes, but now if the caller was expecting one memory type and gets
> >> another one that is something I think the driver would want to know.
> >> At a minimum I don't think we want to get emails about pmem driver
> >> performance problems when someone's platform is silently degrading WB
> >> to UC for example.
> >
> > The pmem driver creates an ioremap map to an NVDIMM range first.  So,
> > there will be no conflict at this point, unless there is a conflicting
> > driver claiming the same NVDIMM range.
> 
> Hmm, I thought it would be WB due to this comment in is_new_memtype_allowed()
> 
>         /*
>          * PAT type is always WB for untracked ranges, so no need to check.
>          */

This comment applies to the ISA range, where ioremap() does not create
any mapping to, i.e. untracked.  You can ignore this comment for NVDIMM.

Thanks,
-Toshi

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