Re: __HAVE_ARCH_PTE_DEVMAP - bug or intended behaviour?

[Robin Murphy <robin.murphy@arm.com>]

On 31/10/2018 20:41, Dan Williams wrote:
> On Wed, Oct 31, 2018 at 10:08 AM Robin Murphy <robin.murphy@arm.com> wrote:
>>
>> Hi mm folks,
>>
>> I'm looking at ZONE_DEVICE support for arm64, and trying to make sense
>> of a build failure has led me down the rabbit hole of pfn_t.h, and
>> specifically __HAVE_ARCH_PTE_DEVMAP in this first instance.
>>
>> The failure itself is a link error in remove_migration_pte() due to a
>> missing definition of pte_mkdevmap(), but I'm a little confused at the
>> fact that it's explicitly declared without a definition, as if that
>> breakage is deliberate.
> 
> It's deliberate, it's only there to allow mm/memory.c to compile. The
> compiler can see that pfn_t_devmap(pfn) is always false in the
> !__HAVE_ARCH_PTE_DEVMAP case and throws away the attempt to link to
> pte_devmap().
> 
> The summary is that an architecture needs to devote a free/software
> pte bit for Linux to indicate "device pfns".

Thanks for the explanation(s), that's been super helpful. So 
essentially, the WIP configuration I currently have 
(ARCH_HAS_ZONE_DEVICE=y but !__HAVE_ARCH_PTE_DEVMAP) is fundamentally 
incomplete, and even if I convince a ZONE_DEVICE=y config to build with 
the devmap stubs, it would end up going wrong in exciting ways at 
runtime - is that the gist of it? If that is the case, then I might also 
have a go at streamlining some of the configs to make those dependencies 
more apparent.

Cheers,
Robin.