Re: [Xen-devel] [RFC XEN PATCH v4 00/41] Add vNVDIMM support to HVM domains

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 13.02.18 at 12:13, <roger.pau@xxxxxxxxxx> wrote:
> On Tue, Feb 13, 2018 at 04:05:45AM -0700, Jan Beulich wrote:
>> >>> On 13.02.18 at 11:29, <roger.pau@xxxxxxxxxx> wrote:
>> > On Tue, Feb 13, 2018 at 03:06:24AM -0700, Jan Beulich wrote:
>> >> >>> On 12.02.18 at 11:05, <roger.pau@xxxxxxxxxx> wrote:
>> >> > If you map the NVDIMM as MMIO to Dom0 you don't need the M2P entries
>> >> > IIRC, and if it's mapped using 1GB pages it shouldn't use that much
>> >> > memory for the page tables (ie: you could just use normal RAM for the
>> >> > page tables that map the NVDIMM IMO). Of course that only applies to
>> >> > PVH/HVM.
>> >> 
>> >> But in order to use (part of) it in a RAM-like manner we need struct
>> >> page_info for it.
>> > 
>> > I guess the main use of this would be to grant NVDIMM pages? And
>> > without a page_info that's not possible.
>> 
>> Why grant? Simply giving such a page as RAM to a guest would
>> already be a problem without struct page_info (as then we can't
>> track the page owner, nor can we refcount the page).
> 
> My point was to avoid doing that, and always assign the pages as
> MMIO, which IIRC doesn't require a struct page_info.

MMIO pages can't be used for things like page tables, because of
the refcounting that's needed. The page being like RAM, however,
implies that the guest needs to be able to use it as anything a RAM
page can be used for.

Jan


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