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Re: [RFC PATCH] xen/memory: Introduce a hypercall to provide unallocated space

Hello all

On 28.07.21 19:18, Oleksandr Tyshchenko wrote:
From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>

Add XENMEM_get_unallocated_space hypercall which purpose is to
query hypervisor to find regions of guest physical address space
which are unused and can be used to create grant/foreign mappings
instead of wasting real pages from the domain memory for
establishing these mappings. The problem with the current Linux
on Xen on Arm behaviour is if we want to map some guest memory
regions in advance or to perform cache mappings in the backend
we might run out of memory in the host (see XSA-300).
This of course, depends on the both host and guest memory sizes.

The "unallocated space" can't be figured out precisely by
the domain on Arm without hypervisor involvement:
- not all device I/O regions are known by the time domain starts
   creating grant/foreign mappings
- the Dom0 is not aware of memory regions used for the identity
   mappings needed for the PV drivers to work
In both cases we might end up re-using these regions by
a mistake. So, the hypervisor which maintains the P2M for the domain
is in the best position to provide "unallocated space".

The arch code is in charge of finding these regions and filling
in corresponding array in new helper arch_get_unallocated_space().

This patch implements common and Arm parts, the x86 specific bits
are left uniplemented for now.

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
Current patch is based on the latest staging branch:
73c932d tools/libxc: use uint32_t for pirq in xc_domain_irq_permission
and also available at:

The corresponding Linux changes you can find at:
I am going to publish them soon.

The Linux changes are already pushed. You can find them at:

1. Small prereq patch


2. Main patch



Oleksandr Tyshchenko



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