Re: [Qemu-devel] [PATCH v2 2/2] virtio-pmem: Add virtio pmem driver

[Pankaj Gupta]

> >
> >
> > > 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