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Re: [PATCH V7 2/2] libxl/arm: Add handling of extended regions for DomU

Hi Oleksandr,

On 14/10/2021 12:40, Oleksandr Tyshchenko wrote:
From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>

The extended region (safe range) is a region of guest physical
address space which is unused and could be safely used to create
grant/foreign mappings instead of wasting real RAM pages from
the domain memory for establishing these mappings.

The extended regions are chosen at the domain creation time and
advertised to it via "reg" property under hypervisor node in
the guest device-tree. As region 0 is reserved for grant table
space (always present), the indexes for extended regions are 1...N.
If extended regions could not be allocated for some reason,
Xen doesn't fail and behaves as usual, so only inserts region 0.

Please note the following limitations:
- The extended region feature is only supported for 64-bit domain
- The ACPI case is not covered.


The algorithm to choose extended regions for non-direct mapped
DomU is simpler in comparison with the algorithm for direct mapped
Dom0. We usually have a lot of unused space above 4GB, and might
have some unused space below 4GB (depends on guest memory size).
Try to allocate separate 2MB-aligned extended regions from the first
(below 4GB) and second (above 4GB) RAM banks taking into the account
the maximum supported guest physical address space size and the amount
of memory assigned to the guest. The minimum size of extended region
the same as for Dom0 (64MB).

Please note, we introduce fdt_property_reg_placeholder helper which
purpose is to create N ranges that are zeroed. The interesting fact
is that libfdt already has fdt_property_placeholder(). But this was
introduced only in 2017, so there is a risk that some distros may not
ship the last libfdt version. This is why we implement our own light
variant for now.

Suggested-by: Julien Grall <jgrall@xxxxxxxxxx>
Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>

Reviewed-by: Julien Grall <jgrall@xxxxxxxxxx>


Julien Grall



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