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

[Oleksandr <olekstysh@xxxxxxxxx>]

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:
> https://github.com/otyshchenko1/xen/commits/map_opt_ml1
>
> The corresponding Linux changes you can find at:
> https://github.com/otyshchenko1/linux/commits/map_opt_ml1
> I am going to publish them soon.
The Linux changes are already pushed. You can find them at:

1. Small prereq patch

https://lore.kernel.org/lkml/1627490656-1267-1-git-send-email-olekstysh@xxxxxxxxx/

2. Main patch

https://lore.kernel.org/lkml/1627490656-1267-2-git-send-email-olekstysh@xxxxxxxxx/

--
Regards,

Oleksandr Tyshchenko