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Re: [PATCH v3 4/4] xen: add helpers to allocate unpopulated memory

On 27.07.20 11:13, Roger Pau Monne wrote:
To be used in order to create foreign mappings. This is based on the
ZONE_DEVICE facility which is used by persistent memory devices in
order to create struct pages and kernel virtual mappings for the IOMEM
areas of such devices. Note that on kernels without support for
ZONE_DEVICE Xen will fallback to use ballooned pages in order to
create foreign mappings.

The newly added helpers use the same parameters as the existing
{alloc/free}_xenballooned_pages functions, which allows for in-place
replacement of the callers. Once a memory region has been added to be
used as scratch mapping space it will no longer be released, and pages
returned are kept in a linked list. This allows to have a buffer of
pages and prevents resorting to frequent additions and removals of

If enabled (because ZONE_DEVICE is supported) the usage of the new
functionality untangles Xen balloon and RAM hotplug from the usage of
unpopulated physical memory ranges to map foreign pages, which is the
correct thing to do in order to avoid mappings of foreign pages depend
on memory hotplug.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
I've not added a new memory_type type and just used
MEMORY_DEVICE_DEVDAX which seems to be what we want for such memory
regions. I'm unsure whether abusing this type is fine, or if I should
instead add a specific type, maybe MEMORY_DEVICE_GENERIC? I don't
think we should be using a specific Xen type at all.

What about introducing MEMORY_DEVICE_VIRT? The comment for
MEMORY_DEVICE_DEVDAX doesn't really fit, as there is no character device

In the end its the memory management maintainers to decide that.

Other than that you can have my

Reviewed-by: Juergen Gross <jgross@xxxxxxxx>




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