On Tue, Nov 22, 2016 at 12:10 PM, Daniel Vetter <daniel@xxxxxxxx> wrote: > On Tue, Nov 22, 2016 at 9:01 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote: >> On Tue, Nov 22, 2016 at 10:59 AM, Serguei Sagalovitch >> <serguei.sagalovitch@xxxxxxx> wrote: >>> I personally like "device-DAX" idea but my concerns are: >>> >>> - How well it will co-exists with the DRM infrastructure / implementations >>> in part dealing with CPU pointers? >> >> Inside the kernel a device-DAX range is "just memory" in the sense >> that you can perform pfn_to_page() on it and issue I/O, but the vma is >> not migratable. To be honest I do not know how well that co-exists >> with drm infrastructure. >> >>> - How well we will be able to handle case when we need to "move"/"evict" >>> memory/data to the new location so CPU pointer should point to the new >>> physical location/address >>> (and may be not in PCI device memory at all)? >> >> So, device-DAX deliberately avoids support for in-kernel migration or >> overcommit. Those cases are left to the core mm or drm. The device-dax >> interface is for cases where all that is needed is a direct-mapping to >> a statically-allocated physical-address range be it persistent memory >> or some other special reserved memory range. > > For some of the fancy use-cases (e.g. to be comparable to what HMM can > pull off) I think we want all the magic in core mm, i.e. migration and > overcommit. At least that seems to be the very strong drive in all > general-purpose gpu abstractions and implementations, where memory is > allocated with malloc, and then mapped/moved into vram/gpu address > space through some magic, but still visible on both the cpu and gpu > side in some form. Special device to allocate memory, and not being > able to migrate stuff around sound like misfeatures from that pov. Agreed. For general purpose P2P use cases where all you want is direct-I/O to a memory range that happens to be on a PCIe device then I think a special device fits the bill. For gpu P2P use cases that already have migration/overcommit expectations then it is not a good fit. -- To unsubscribe from this list: send the line "unsubscribe linux-pci" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
