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

[Dan Williams]

On Fri, Oct 12, 2018 at 10:01 PM Pankaj Gupta <address@hidden> 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.