Re: [Xen-devel] [RFC Design Doc] Add vNVDIMM support for Xen

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 22.04.16 at 14:54, <haozhong.zhang@xxxxxxxxx> wrote:
> On 04/22/16 06:36, Jan Beulich wrote:
>> >>> On 22.04.16 at 14:26, <haozhong.zhang@xxxxxxxxx> wrote:
>> > On 04/22/16 04:53, Jan Beulich wrote:
>> >> Perhaps I have got confused by the back and forth. If we're to
>> >> use struct page_info, then everything should be following a
>> >> similar flow to what happens for normal RAM, i.e. normal page
>> >> allocation, and normal assignment of pages to guests.
>> >>
>> > 
>> > I'll follow the normal assignment of pages to guests for pmem, but not
>> > the normal page allocation. Because allocation is difficult to always
>> > get the same pmem area for the same guest every time. It still needs
>> > input from others (e.g. toolstack) that can provide the exact address.
>> 
>> Understood.
>> 
>> > Because the address is now not decided by xen hypervisor, certain
>> > permission track is needed. For this part, we will re-use the existing
>> > one for MMIO. Directly using existing range struct for pmem may
>> > consume too much space, so I proposed to choose different data
>> > structures or put limitation on exiting range struct to avoid or
>> > mitigate this problem.
>> 
>> Why would these consume too much space? I'd expect there to be
>> just one or very few chunks, just like is the case for MMIO ranges
>> on devices.
> 
> As Ian Jackson indicated [1], there are several cases that a pmem page
> can be accessed from more than one domains. Then every domain involved
> needs a range struct to track its access permission to that pmem
> page. In a worst case, e.g. the first of every two contiguous pages on
> a pmem are assigned to a domain and are shared with all other domains,
> though the size of range struct for a single domain maybe acceptable,
> the total will still be very large.

Everything Ian has mentioned there is what normal RAM pages
also can get used for, yes as you have yourself said (still visible
in context above) you mean to only do allocation differently. Hence
the permission tracking you talk of should be necessary only for the
owning domain (to get validated during allocation), everything else
should follow the normal life cycle of a RAM page.

Jan


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