> > > > > > > On Fri, Oct 12, 2018 at 10:01 PM Pankaj Gupta <pagupta@xxxxxxxxxx> wrote: > > > > > > > > This patch adds virtio-pmem driver for KVM guest. > > > > > > > > Guest reads the persistent memory range information from > > > > Qemu over VIRTIO and registers it on nvdimm_bus. It also > > > > creates a nd_region object with the persistent memory > > > > range information so that existing 'nvdimm/pmem' driver > > > > can reserve this into system memory map. This way > > > > 'virtio-pmem' driver uses existing functionality of pmem > > > > driver to register persistent memory compatible for DAX > > > > capable filesystems. > > > > > > > > This also provides function to perform guest flush over > > > > VIRTIO from 'pmem' driver when userspace performs flush > > > > on DAX memory range. > > > > > > Before we can move forward with this driver we need additional > > > filesystem enabling to detect when the backing device is fronting DAX > > > pmem or a paravirtualized page cache through virtio-pmem. Any > > > interface that requires fsync() and a round trip to the hypervisor to > > > flush host page cache is not DAX. > > > > I saw your proposal[1] for new mmap flag MAP_DIRECT. IIUIC mapping should > > fail for > > MAP_DIRECT if it requires explicit flush or buffer indirection. So, if we > > disable > > MAP_SYNC flag for virtio-pmem this should fail MAP_DIRECT as well? > > Otherwise > > without MAP_DIRECT, virtio-pmem should be defaulted to VIRTIO flush > > mechanism. > > Right, although I wouldn't worry about MAP_DIRECT in the short term > since we're still discussing what the upstream interface. Regardless > of whether MAP_DIRECT is specified or not the virtio-flush mechanism > would always be used for virtio-pmem. I.e. there is no possibility to > get full DAX operation with virtio-pmem, only the page-cache bypass > sub-set. Agree. I will also follow the thread. > > Taking a look at where we could inject this check for filesystems it's > a bit awkward to do it in xfs_file_mmap() for example because we do > not have the backing device for the extents of the inode. So at a > minimum you would need to investigate calling xfs_inode_supports_dax() > from that path and teaching it about a new dax_device flag. I'm > thinking the dax_device flag should be called DAXDEV_BUFFERED to > indicate the presence of software buffering on a device that otherwise > supports bypassing the local page cache. Sure. Will investigate XFS code as suggested. Thanks for the detail directions towards the solution. Will try to come up with a solution. Best regards, Pankaj
