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Re: [Qemu-devel] KVM "fake DAX" flushing interface - discussion

From: Pankaj Gupta
Subject: Re: [Qemu-devel] KVM "fake DAX" flushing interface - discussion
Date: Mon, 6 Nov 2017 02:57:20 -0500 (EST)

> [..]
> >> Yes, the GUID will specifically identify this range as "Virtio Shared
> >> Memory" (or whatever name survives after a bikeshed debate). The
> >> libnvdimm core then needs to grow a new region type that mostly
> >> behaves the same as a "pmem" region, but drivers/nvdimm/pmem.c grows a
> >> new flush interface to perform the host communication. Device-dax
> >> would be disallowed from attaching to this region type, or we could
> >> grow a new device-dax type that does not allow the raw device to be
> >> mapped, but allows a filesystem mounted on top to manage the flush
> >> interface.
> >
> >
> > I am afraid it is not a good idea that a single SPA is used for multiple
> > purposes. For the region used as "pmem" is directly mapped to the VM so
> > that guest can freely access it without host's assistance, however, for
> > the region used as "host communication" is not mapped to VM, so that
> > it causes VM-exit and host gets the chance to do specific operations,
> > e.g, flush cache. So we'd better distinctly define these two regions to
> > avoid the unnecessary complexity in hypervisor.
> Good point, I was assuming that the mmio flush interface would be
> discovered separately from the NFIT-defined memory range. Perhaps via
> PCI in the guest? This piece of the proposal  needs a bit more
> thought...

Also, in earlier discussions we agreed for entire device flush whenever guest
performs a fsync on DAX file. If we do a MMIO call for this, guest CPU would be
trapped for the duration device flush is completed.

Instead, if we do perform an asynchronous flush guest CPU's can be utilized by
some other tasks till flush completes?


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