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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 28/07/2020 17:59, Roger Pau Monné wrote:
> On Tue, Jul 28, 2020 at 05:48:23PM +0100, Julien Grall wrote:
>> Hi,
>>
>> On 27/07/2020 10:13, 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
>>> regions.
>>>
>>> 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.
>> I think this is going to break Dom0 on Arm if the kernel has been built with
>> hotplug. This is because you may end up to re-use region that will be used
>> for the 1:1 mapping of a foreign map.
>>
>> Note that I don't know whether hotplug has been tested on Xen on Arm yet. So
>> it might be possible to be already broken.
>>
>> Meanwhile, my suggestion would be to make the use of hotplug in the balloon
>> code conditional (maybe using CONFIG_ARM64 and CONFIG_ARM)?
> Right, this feature (allocation of unpopulated memory separated from
> the balloon driver) is currently gated on CONFIG_ZONE_DEVICE, which I
> think could be used on Arm.
>
> IMO the right solution seems to be to subtract the physical memory
> regions that can be used for the identity mappings of foreign pages
> (all RAM on the system AFAICT) from iomem_resource, as that would make
> this and the memory hotplug done in the balloon driver safe?

The right solution is a mechanism for translated guests to query Xen to
find regions of guest physical address space which are unused, and can
be safely be used for foreign/grant/other  mappings.

Please don't waste any more time applying more duct tape to a broken
system, and instead spend the time organising some proper foundations.

~Andrew