On 04/03/2019 09:37 PM, Jerome Glisse wrote: > 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. Driver managed ZONE_DEVICE memory which never maps into user page table is not a valid use case for ZONE_DEVICE ? Also what about MEMORY_DEVICE_PRIVATE ? That can never be mapped into user page table. A driver can still manage these non coherent memory through it's struct pages (which will be allocated inside RAM) > 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. CONFIG_ZONE_DEVICE does not do that already ! All it says is that a device memory range can be plugged into ZONE_DEVICE either as PRIVATE (non-coherent) or PUBLIC/PCI_P2PDMA (coherent) memory without mandating anything about how these memory will be further used. > > What is the end use case you are looking for ? Persistent memory ? Persistent memory is one of the primary use cases.
