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

[Dan Williams <dan.j.williams@intel.com>]

On Tue, Apr 4, 2017 at 10:00 AM, Konrad Rzeszutek Wilk
<konrad.wilk@oracle.com> wrote:
> On Sat, Apr 01, 2017 at 08:45:45AM -0700, Dan Williams wrote:
>> On Sat, Apr 1, 2017 at 4:54 AM, Konrad Rzeszutek Wilk <konrad@darnok.org> wrote:
>> > ..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).
>>
>> So, I'm confused again. This patchset / enabling requires both M2P and
>> contiguous PMEM ranges. If the PMEM is contiguous it seems you don't
>> need M2P and can just reuse the MMIO enabling, or am I missing
>> something?
>
> I think I am confusing you.
>
> The patchset (specifically 04/15] xen/x86: add XEN_SYSCTL_nvdimm_pmem_setup to setup host pmem )
> adds a hypercall to tell Xen where on the NVDIMM it can put
> the M2P array and as well the frametables ('struct page').
>
> There is no range support. The reason is that if break up
> an NVDIMM in various chunks (and then put a filesystem on top of it) - and
> then figure out which of the SPAs belong to the file - and then
> "expose" that file to a guest as NVDIMM - it's SPAs won't
> be contingous. Hence the hypervisor would need to break down
> the 'ranges' structure down in either a bitmap or an M2P
> and also store it. This can get quite tricky so you may
> as well just start with an M2P and 'struct page'.

Ok... but the problem then becomes that the filesystem is free to
change the file-offset to SPA mapping any time it wants. So the M2P
support is broken if it expects static relationships.

> The placement of those datastructures is "
> v2 patch
>    series relies on users/admins in Dom0 instead of Dom0 driver to indicate the
>    location to store the frametable and M2P of pmem.
> "
>
> Hope this helps?

It does, but it still seems we're stuck between either 1/ not needing
M2P if we can pass a whole pmem-namespace through to the guest or 2/
M2P being broken by non-static file-offset to physical address
mappings.

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