Re: [Qemu-devel] [RFC 2/2] KVM: add virtio-pmem driver

[Christoph Hellwig <hch-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>]

On Thu, Oct 19, 2017 at 11:21:26AM -0700, Dan Williams wrote:
> The difference is that nvdimm_flush() is not mandatory, and that the
> platform will automatically perform the same flush at power-fail.
> Applications should be able to assume that if they are using MAP_SYNC
> that no other coordination with the kernel or the hypervisor is
> necessary.
> 
> Advertising this as a generic Persistent Memory range to the guest
> means that the guest could theoretically use it with device-dax where
> there is no driver or filesystem sync interface. The hypervisor will
> be waiting for flush notifications and the guest will just issue cache
> flushes and sfence instructions. So, as far as I can see we need to
> differentiate this virtio-model from standard "Persistent Memory" to
> the guest and remove the possibility of guests/applications making the
> wrong assumption.

So add a flag that it is not.  We already have the nd_volatile type,
that is special.  For now only in Linux, but I think adding this type
to the spec eventually would be very useful for efficiently exposing
directly mappable device to VM guests.