On Tue, Jan 30, 2018 at 10:02 PM, Haozhong Zhang <haozhong.zh...@intel.com> wrote: > Linux 4.15 introduces a new mmap flag MAP_SYNC, which can be used to > guarantee the write persistence to mmap'ed files supporting DAX (e.g., > files on ext4/xfs file system mounted with '-o dax').
Wait, MAP_SYNC does not guarantee persistence. It makes sure that the metadata is in sync after a fault. However, that does not make filesystem-DAX safe for use with QEMU, because we still need to coordinate DMA with fileystem operations. There is no way to do that coordination from within a guest. QEMU needs to use device-dax if the guest might ever perform DMA to a virtual-pmem range. See this patch set for more details on the DAX vs DMA problem [1]. I think we need to enforce this in the host kernel. I.e. do not allow file backed DAX pages to be mapped in EPT entries unless / until we have a solution to the DMA synchronization problem. Apologies for not noticing this earlier. [1]: https://lists.01.org/pipermail/linux-nvdimm/2017-December/013704.html
