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

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