On 31/10/2018 20:41, Dan Williams wrote:
On Wed, Oct 31, 2018 at 10:08 AM Robin Murphy <robin.murphy@xxxxxxx> 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.
