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

[Haozhong Zhang <haozhong.zhang@intel.com>]

On 03/29/16 04:49, Jan Beulich wrote:
> >>> On 29.03.16 at 12:10, <haozhong.zhang@intel.com> wrote:
> > On 03/29/16 03:11, Jan Beulich wrote:
> >> >>> On 29.03.16 at 10:47, <haozhong.zhang@intel.com> wrote:
[..]
> >> > I still cannot find a neat approach to manage guest permissions for
> >> > nvdimm pages. A possible one is to use a per-domain bitmap to track
> >> > permissions: each bit corresponding to an nvdimm page. The bitmap can
> >> > save lots of spaces and even be stored in the normal ram, but
> >> > operating it for a large nvdimm range, especially for a contiguous
> >> > one, is slower than rangeset.
> >> 
> >> I don't follow: What would a single bit in that bitmap mean? Any
> >> guest may access the page? That surely wouldn't be what we
> >> need.
> >>
> > 
> > For a host having a N pages of nvdimm, each domain will have a N bits
> > bitmap. If the m'th bit of a domain's bitmap is set, then that domain
> > has the permission to access the m'th host nvdimm page.
> 
> Which will be more overhead as soon as there are enough such
> domains in a system.
>

Sorry for the late reply.

I think we can make some optimization to reduce the space consumed by
the bitmap.

A per-domain bitmap covering the entire host NVDIMM address range is
wasteful especially if the actual used ranges are congregated. We may
take following ways to reduce its space.

1) Split the per-domain bitmap into multiple sub-bitmap and each
   sub-bitmap covers a smaller and contiguous sub host NVDIMM address
   range. In the beginning, no sub-bitmap is allocated for the
   domain. If the access permission to a host NVDIMM page in a sub
   host address range is added to a domain, only the sub-bitmap for
   that address range is allocated for the domain. If access
   permissions to all host NVDIMM pages in a sub range are removed
   from a domain, the corresponding sub-bitmap can be freed.

2) If a domain has access permissions to all host NVDIMM pages in a
   sub range, the corresponding sub-bitmap will be replaced by a range
   struct. If range structs are used to track adjacent ranges, they
   will be merged into one range struct. If access permissions to some
   pages in that sub range are removed from a domain, the range struct
   should be converted back to bitmap segment(s).

3) Because there might be lots of above bitmap segments and range
   structs per-domain, we can organize them in a balanced interval
   tree to quickly search/add/remove an individual structure.

In the worst case that each sub range has non-contiguous pages
assigned to a domain, above solution will use all sub-bitmaps and
consume more space than a single bitmap because of the extra space for
organization. I assume that the sysadmin should be responsible to
ensure the host nvdimm ranges assigned to each domain as contiguous
and congregated as possible in order to avoid the worst case. However,
if the worst case does happen, xen hypervisor should refuse to assign
nvdimm to guest when it runs out of memory.

Haozhong

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