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Re: [RFC PATCH 0/3] Add handling of extended regions (safe ranges) on Arm (Was "xen/memory: Introduce a hypercall to provide unallocated space")

Hello all

On 10.09.21 02:20, Oleksandr wrote:

Hello all

On 07.09.21 20:09, Oleksandr Tyshchenko wrote:
From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>

You can find an initial discussion at [1].

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.

The extended regions are calculated differently for direct mapped
Dom0 (with and without IOMMU) and non-direct mapped DomUs.

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

Also please note that we haven't figured out yet how to properly
extend the Xen hypervisor device-tree bindings on Arm (either via new
compatible or via new property). I decided to go with new property
for now, but this can be changed. This uncertainty is the main reason
why this series is marked as RFC.

Sorry, I messed up the device tree binding's purpose here.

New DT property "extended-region" (to inform guest about the presence of additional regions in "reg" property) is not really needed. Guest can simply infer that from the number of regions in "reg" property (region 0 - reserved for grant table space, regions 1...n - extended regions). Instead, new compatible/property will be needed (but only after this patch [1] or alternative goes in) to indicate that "region 0 is safe to use". Until this patch is merged it is
not safe to use extended regions for the grant table space.

Thanks to Julien for clarifying these bits.

I am going to remove the advertisement of unneeded "extended-region" property in the code and send new version soon.

I have just pushed new version [1]. Please ignore this one. Sorry for the inconvenience.

[1] https://lore.kernel.org/xen-devel/1631297924-8658-1-git-send-email-olekstysh@xxxxxxxxx/


Oleksandr Tyshchenko



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