On Wed, Apr 03, 2019 at 02:58:28PM +0100, Robin Murphy wrote: > [ +Dan, Jerome ] > > On 03/04/2019 05:30, Anshuman Khandual wrote: > > Arch implementation for functions which create or destroy vmemmap mapping > > (vmemmap_populate, vmemmap_free) can comprehend and allocate from inside > > device memory range through driver provided vmem_altmap structure which > > fulfils all requirements to enable ZONE_DEVICE on the platform. Hence just > > ZONE_DEVICE is about more than just altmap support, no? > > > enable ZONE_DEVICE by subscribing to ARCH_HAS_ZONE_DEVICE. But this is only > > applicable for ARM64_4K_PAGES (ARM64_SWAPPER_USES_SECTION_MAPS) only which > > creates vmemmap section mappings and utilize vmem_altmap structure. > > What prevents it from working with other page sizes? One of the foremost > use-cases for our 52-bit VA/PA support is to enable mapping large quantities > of persistent memory, so we really do need this for 64K pages too. FWIW, it > appears not to be an issue for PowerPC. > > > Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx> > > --- > > arch/arm64/Kconfig | 1 + > > 1 file changed, 1 insertion(+) > > > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig > > index db3e625..b5d8cf5 100644 > > --- a/arch/arm64/Kconfig > > +++ b/arch/arm64/Kconfig > > @@ -31,6 +31,7 @@ config ARM64 > > select ARCH_HAS_SYSCALL_WRAPPER > > select ARCH_HAS_TEARDOWN_DMA_OPS if IOMMU_SUPPORT > > select ARCH_HAS_TICK_BROADCAST if GENERIC_CLOCKEVENTS_BROADCAST > > + select ARCH_HAS_ZONE_DEVICE if ARM64_4K_PAGES > > IIRC certain configurations (HMM?) don't even build if you just turn this on > alone (although of course things may have changed elsewhere in the meantime) > - crucially, though, from previous discussions[1] it seems fundamentally > unsafe, since I don't think we can guarantee that nobody will touch the > corners of ZONE_DEVICE that also require pte_devmap in order not to go > subtly wrong. I did get as far as cooking up some patches to sort that out > [2][3] which I never got round to posting for their own sake, so please > consider picking those up as part of this series. Correct _do not_ enable ZONE_DEVICE without support for pte_devmap detection. If you want some feature of ZONE_DEVICE. Like HMM as while DAX does require pte_devmap, HMM device private does not. So you would first have to split ZONE_DEVICE into more sub-features kconfig option. What is the end use case you are looking for ? Persistent memory ? Cheers, Jérôme
