Re: [Xen-devel] [RFC XEN PATCH v2 00/15] Add vNVDIMM support to HVM domains

[Konrad Rzeszutek Wilk <konrad@xxxxxxxxxx>]

..snip..
> >> Is there a resource I can read more about why the hypervisor needs to
> >> have this M2P mapping for nvdimm support?
> >
> > M2P is basically an array of frame numbers. It's indexed by the host
> > page frame number, or the machine frame number (MFN) in Xen's
> > definition. The n'th entry records the guest page frame number that is
> > mapped to MFN n. M2P is one of the core data structures used in Xen
> > memory management, and is used to convert MFN to guest PFN. A
> > read-only version of M2P is also exposed as part of ABI to guest. In
> > the previous design discussion, we decided to put the management of
> > NVDIMM in the existing Xen memory management as much as possible, so
> > we need to build M2P for NVDIMM as well.
> >
> 
> Thanks, but what I don't understand is why this M2P lookup is needed?

Xen uses it to construct the EPT page tables for the guests.

> Does Xen establish this metadata for PCI mmio ranges as well? What Xen

It doesn't have that (M2P) for PCI MMIO ranges. For those it has an
ranges construct (since those are usually contingous and given
in ranges to a guest).
> memory management operations does this enable? Sorry if these are
> basic Xen questions, I'm just looking to see if we can make the
> mapping support more dynamic. For example, what if we wanted to change
> the MFN to guest PFN relationship after every fault?

As in swap it out? (Like a hard drive swaps out faulty sectors?).
That is certainly done. We also have tools (xen-hptool) that can
mark certain pages as broken/etc and inject the MCEs in the guest
to reflect that. But all of that is driven by hypercalls

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